A simple cyst on the kidney is a round, fluid-filled sac that forms on or within the kidney. These cysts are almost always benign, meaning they have no malignant potential and are not a form of kidney cancer. Most people who have one never know it exists until it shows up incidentally on an imaging scan done for another reason.
Simple kidney cysts are extremely common, especially as you get older. About 23% of people in their 50s have at least one, and that number climbs to roughly 33% by age 60 and beyond. They’re one of the most frequent incidental findings on abdominal imaging, and in the vast majority of cases, they require no treatment at all.
How Simple Kidney Cysts Form
The exact cause isn’t fully understood, but simple cysts develop when a small area of the kidney’s tubules (the tiny tubes that filter and collect urine) becomes blocked or weakened, allowing fluid to pool and form a sac. The fluid inside is typically clear or straw-colored and similar in density to water. Unlike polycystic kidney disease, which is a genetic condition that produces many cysts and progressively damages kidney function, simple cysts are isolated findings that don’t signal an inherited disorder.
Age is the strongest risk factor. The kidney’s tissue gradually changes over decades, making these small fluid pockets more likely to develop. Men tend to develop them slightly more often than women, though the reason for this isn’t clear.
What a Simple Cyst Looks Like on Imaging
Most simple cysts are discovered on an ultrasound or CT scan. On ultrasound, they have a very distinctive appearance: a completely dark (fluid-filled) center, a wall so thin it’s barely visible, and no internal walls, debris, or solid material inside. These features make them easy for radiologists to identify with confidence.
Doctors classify kidney cysts using a system called the Bosniak classification, which ranges from Category I (clearly benign) to Category IV (likely cancerous). A simple cyst falls into Category I: water-density fluid, thin smooth walls, sharp borders, and no enhancement when contrast dye is injected. Category II cysts are still considered benign but may have a thin internal wall or tiny calcifications. Both Category I and II cysts require no further imaging or follow-up, because their risk of being cancerous is essentially zero.
If a cyst has thicker walls, irregular borders, multiple internal divisions, or areas that light up with contrast dye, it moves into higher Bosniak categories and may need monitoring or further evaluation. But a cyst labeled “simple” on your imaging report has already been placed in the lowest risk category.
Symptoms and When Cysts Cause Problems
The large majority of simple kidney cysts produce no symptoms whatsoever. You can have one for years, or even your entire life, without ever feeling it. Problems only arise in rare cases when a cyst grows large enough to press on surrounding structures. When that happens, symptoms can include dull pain in your back or side, a sense of fullness or discomfort in your upper abdomen, or, less commonly, high blood pressure if the cyst compresses blood vessels near the kidney.
In rare instances, a cyst can become infected, causing fever, pain, and tenderness over the kidney area. A cyst can also rupture, which typically causes sudden sharp pain and sometimes blood in the urine. These complications are uncommon enough that most people with simple cysts will never experience them.
How Fast Simple Cysts Grow
Simple cysts do tend to grow slowly over time. One longitudinal study tracking cysts in adults found an average growth rate of about 4.4 millimeters per year. Interestingly, cysts in people younger than 50 grew faster (roughly 5.7 mm per year) than those in people 50 and older (about 3.7 mm per year). People with multiple cysts or cysts in both kidneys also saw slightly faster growth compared to those with a single cyst on one side.
This growth is gradual enough that even a cyst tracked over several years typically remains small and harmless. Growth alone doesn’t change the diagnosis or make a simple cyst dangerous. It’s the characteristics of the cyst, not its size, that determine whether it needs attention.
Do Simple Cysts Need Treatment?
If your cyst isn’t causing symptoms, the answer is no. Category I simple cysts have no malignant potential, and guidelines from urology organizations state that no follow-up imaging is required. Your doctor may note it on your chart, but there’s nothing you need to do about it.
Treatment only becomes relevant when a cyst is large enough to cause pain, obstruct urine flow, or create other symptoms. In those cases, the two main options are drainage and sclerotherapy.
Simple aspiration involves inserting a needle through the skin (guided by ultrasound) to drain the fluid. The problem with aspiration alone is that cysts almost always refill. Studies show recurrence rates near 90% within just 12 weeks when only drainage is performed. To prevent this, doctors often follow drainage with sclerotherapy, injecting a solution (typically alcohol) into the emptied cyst to destroy its lining so it can’t produce more fluid. This combination has a success rate above 90%, with recurrence dropping to under 10% at six months.
For very large or awkwardly positioned cysts, a minimally invasive surgical procedure can remove or permanently open the cyst wall. This is relatively uncommon and reserved for cases where simpler approaches haven’t worked or the cyst is causing significant problems.
Simple Cysts and Kidney Function
A simple cyst, even a moderately large one, does not damage your kidneys or lead to kidney disease. The surrounding kidney tissue continues to function normally. This is a key distinction from polycystic kidney disease, where hundreds of cysts gradually replace healthy tissue and impair the organ’s ability to filter blood. If your imaging report says “simple cyst,” it’s not an early sign of polycystic disease or any progressive kidney condition.
If you’ve been told you have a simple kidney cyst, the most important takeaway is that it’s one of the most common and least concerning findings in medical imaging. For the vast majority of people, it will never cause symptoms, never require treatment, and never affect kidney health.