Kidney Cysts: What They Are, When to Worry, and How They're Treated

A kidney cyst is a fluid-filled sac that forms on or within the kidney. Simple kidney cysts, the most common type, are among the most frequently encountered incidental findings in radiology: discovered on scans ordered for something entirely unrelated, often in middle-aged and older adults who have had no symptoms whatsoever.

That said, not all kidney cysts are created equal. Some represent a hereditary condition that can lead to kidney failure. Others, while benign in appearance, require monitoring because a small percentage may harbor cancer.

"Simple renal cysts are extremely common and their incidence increases with age — approximately 50% of people aged 50 and older have at least one. Most are asymptomatic and require no treatment." — StatPearls / NIH, Simple Renal Cyst (2026)

Kidney Cysts by the Numbers

• 50% of adults over 50 have at least one simple kidney cyst
• 25% of adults over 40 have simple kidney cysts
• 600,000 Americans living with polycystic kidney disease (PKD)
• 12 million people worldwide affected by ADPKD — the most common genetic PKD

What Exactly Is a Kidney Cyst?

The kidneys are a pair of bean-shaped organs responsible for filtering blood, removing waste products, and regulating fluid balance. Kidney cysts are abnormal, round or oval-shaped sacs — typically filled with fluid — that form on the surface or within the tissue of one or both kidneys.

Most simple cysts have a thin, smooth wall with no internal structures, solid components, or calcifications. When a cyst deviates from this simple profile — developing internal walls, calcifications, or solid areas — it may require closer monitoring or biopsy.

The vast majority of simple kidney cysts require no treatment and no surgery. They are typically found incidentally and monitored periodically.

Types of Kidney Cysts

Simple renal cyst (Most common) — A single fluid-filled sac with a thin, smooth wall and no internal features. Not inherited — develops with age. Almost always benign. Found in 50% of adults over 50.

Autosomal dominant PKD (ADPKD) (Genetic) — The most common form of polycystic kidney disease. Caused by mutations in PKD1 or PKD2 genes. Multiple cysts grow in both kidneys over time, progressively impairing kidney function. Affects up to 12 million people worldwide. Symptoms typically appear at ages 30–40.

Autosomal recessive PKD (ARPKD) (Genetic) — A rarer, more severe form that typically manifests in infancy or childhood. Caused by mutations in the PKHD1 gene. Often affects the liver as well as the kidneys. Both parents must carry the gene mutation.

Acquired cystic kidney disease (ACKD) (Acquired) — Develops in people with chronic kidney disease (CKD) or those on long-term dialysis. Carries a higher risk of kidney cancer than simple cysts and requires regular monitoring.

Complex renal cysts — Cysts that have internal walls (septa), calcifications, thickened walls, or solid components. Classified on the Bosniak scale. Some are benign variants; others may harbor cancer.

Medullary sponge kidney & other rare types — Several rarer genetic conditions cause cysts in specific parts of the kidney, including nephronophthisis and Von Hippel-Lindau syndrome (which also increases kidney cancer risk).

The Bosniak Classification

When a kidney cyst is found on imaging, radiologists use the Bosniak classification to categorize its characteristics and estimate malignancy likelihood:

• Category I — Simple benign cyst. Thin smooth wall, no septa or calcifications. Malignancy risk: nearly 0%. No follow-up required.
• Category II — Minimally complex, benign. May have a few thin septa or fine calcifications. Malignancy risk: ~2%. No follow-up required.
• Category IIF — Requires follow-up imaging. More complex than Category II. Malignancy risk: ~6%. Periodic imaging surveillance recommended.
• Category III — Indeterminate. Thickened, irregular septa with measurable enhancement. Malignancy risk: ~50%. Surgery or close surveillance typically recommended.
• Category IV — Malignant. Contains clearly enhancing solid components. Malignancy risk: 90%+. Surgical removal strongly recommended.

Symptoms of Kidney Cysts

Simple kidney cysts are almost always asymptomatic. Symptoms become more likely as cysts grow large or in PKD.

• Dull, aching pain in the back, side (flank), or upper abdomen
• Blood in the urine (hematuria) — can occur if a cyst bleeds or ruptures
• Fever and localized pain — may indicate an infected cyst requiring prompt attention
• High blood pressure — particularly in PKD, where cysts compress kidney blood vessels
• Urinary tract infections — recurring UTIs may occur if cysts become infected
• Declining kidney function — in PKD, cysts replace healthy kidney tissue over time
• Abdominal fullness or swelling — in advanced PKD, kidneys can grow dramatically

Sudden, severe flank pain alongside fever and vomiting may indicate a ruptured or infected cyst — particularly in people with known PKD. This requires prompt medical evaluation.

Complications of Polycystic Kidney Disease (PKD)

Kidney failure — PKD is the 4th leading cause of kidney failure globally, causing about 5% of all end-stage renal disease in the U.S. About half of ADPKD patients reach kidney failure by age 60.

Hypertension — High blood pressure is nearly universal in PKD and often appears years before kidney function declines.

Intracranial aneurysms — About 8–10% of ADPKD patients develop aneurysms in brain blood vessels. Rupture can cause life-threatening hemorrhagic stroke.

Liver cysts — The most common extrarenal complication of ADPKD. Typically benign but can cause discomfort and abdominal distension.

Heart valve abnormalities — About 25% of PKD patients have mitral valve prolapse, which warrants cardiac evaluation.

Chronic pain — Back and flank pain from enlarging kidneys, cyst hemorrhage, or infection is one of the most persistent quality-of-life challenges in PKD.

How Kidney Cysts Are Diagnosed

Ultrasound — The most common first-line imaging tool. Can clearly differentiate simple fluid-filled cysts from solid masses or complex cysts. Radiation-free and widely available.

CT scan — Provides detailed characterization of cyst features including wall thickness, septa, calcifications, and contrast enhancement. Essential for Bosniak classification.

MRI — Superior soft tissue contrast without radiation. Increasingly used for complex cyst evaluation and monitoring of PKD. Can detect subtle enhancement not visible on CT.

Blood and urine tests — Kidney function tests (creatinine, eGFR, BUN) assess whether cysts are affecting kidney function. Genetic testing may be recommended if PKD is suspected.

Biopsy — For Bosniak III or IV cysts where cancer cannot be excluded on imaging, surgical removal or biopsy may be needed.

Treatment Options

For simple kidney cysts — the most common type — no treatment is needed. Periodic imaging (typically every 1–2 years) may be recommended to confirm stability.

Active surveillance / watchful waiting — Standard management for Bosniak I and II cysts. No intervention unless symptoms develop or complexity increases.

Sclerotherapy (aspiration and drainage) — A needle drains the cyst fluid and a sclerosing agent is injected to collapse the cyst walls. Used for symptomatic simple cysts causing pain or obstruction.

Laparoscopic cyst decortication (surgery) — For large, symptomatic, or recurring cysts. A minimally invasive procedure removes the outer wall of the cyst.

Tolvaptan for ADPKD — A vasopressin receptor antagonist approved specifically for slowing kidney growth in adults with ADPKD at risk of rapid progression. The first targeted treatment for PKD.

Blood pressure management — Rigorous blood pressure control — typically with ACE inhibitors or ARBs — is one of the most important interventions in PKD.

Dialysis and kidney transplant — For PKD patients who progress to end-stage renal disease. Kidney transplantation is the preferred treatment for eligible patients. Transplanted kidneys do not develop new PKD cysts.

An infected kidney cyst is a medical emergency. Cyst infections in PKD patients can be difficult to treat because many antibiotics penetrate cyst fluid poorly. Seek medical care promptly — delayed treatment can lead to sepsis.

What to Do If a Kidney Cyst Is Found

If a kidney cyst is discovered incidentally, the most important first step is not to panic — and to get an accurate characterization. A simple cyst finding is almost always good news. A complex or indeterminate finding warrants follow-up with a urologist or nephrologist.

If you have a personal or family history of polycystic kidney disease, or if you are experiencing symptoms like flank pain, hematuria, high blood pressure at a young age, or recurrent urinary infections, speak with your doctor about PKD testing.

iHealth Network's doctor directory can help you find a nephrologist or urologist near you.

This article is for informational purposes only. Sources: Cleveland Clinic — Kidney Cysts (November 2025); StatPearls / NIH — Simple Renal Cyst (2026); National Kidney Foundation — PKD (2026); Mayo Clinic — Kidney Cysts (February 2026).