A renal cyst is a round, fluid-filled sac that forms on or inside a kidney. Most are simple cysts, completely benign and harmless, found in roughly half of all adults over 50 during routine imaging. They rarely cause symptoms, almost never require treatment, and are typically discovered by accident when a doctor orders a scan for something else entirely.
How Renal Cysts Form
Simple renal cysts develop from tiny outpouchings along the kidney’s drainage tubes, called tubular diverticula. These outpouchings become more numerous as you age. Over decades, some gradually fill with fluid and balloon outward, progressing from microscopic pockets to cysts large enough to show up on imaging. This is why simple cysts are rare in people under 20 but increasingly common with each passing decade.
The fluid inside a simple cyst is typically clear or straw-colored, similar to blood plasma that has been filtered by the kidney. The cyst wall is thin and smooth, with no solid tissue or internal structures. This simplicity is exactly what distinguishes a harmless cyst from one that needs closer attention.
Simple vs. Complex Cysts
Not all kidney cysts are identical. Doctors classify them using a system called the Bosniak classification, which ranges from category I (clearly benign) to category IV (likely cancerous). The distinction comes down to a few key features visible on imaging: wall thickness, internal dividers called septa, and whether any solid components are present.
- Bosniak I: A perfectly simple cyst. Thin, smooth walls under 2 mm, no internal dividers, no calcifications. These are benign and need no follow-up.
- Bosniak II: Still benign, but with minor complexity, such as one to three thin internal dividers or small calcifications. No follow-up imaging needed.
- Bosniak IIF: Slightly more complex features like minimally thickened walls or multiple thin dividers. These carry a small but real risk and require monitoring.
- Bosniak III: Thick walls or irregular dividers that raise suspicion for cancer. About 50% of these turn out to be malignant.
- Bosniak IV: Solid nodules or tissue masses within the cyst. These are treated as likely cancerous.
The vast majority of kidney cysts fall into categories I and II. If your imaging report mentions a “simple renal cyst,” that’s a Bosniak I, and it’s nothing to worry about.
Symptoms and Size
Most renal cysts cause no symptoms at all. The typical cyst stays under 2 inches (about 5 cm) in diameter, sitting quietly on the kidney without interfering with anything. You could have one for years and never know.
In rare cases, a cyst grows large enough to press against nearby organs or stretch the kidney’s outer capsule. When that happens, you might notice a dull ache or fullness in your side or upper abdomen. Very large cysts can occasionally cause high blood pressure by compressing kidney tissue, or lead to urinary issues if they press on the drainage system. Infection or bleeding into a cyst, while uncommon, can cause sudden sharp pain and fever.
How Fast They Grow
A 10-year study tracking simple cysts found they grow slowly, averaging just 1.6 mm per year, or about 3.9% of their size annually. Growth was fastest in people in their 40s, where cysts expanded by roughly 3.7 mm per year, then gradually slowed in older age groups. At that pace, a 2 cm cyst would take decades to reach a size that might cause symptoms. This slow, predictable growth is one reason doctors feel comfortable leaving most simple cysts alone.
How Renal Cysts Are Diagnosed
Most kidney cysts are spotted incidentally on an ultrasound, CT scan, or MRI ordered for an unrelated reason, such as abdominal pain, a kidney stone workup, or a general health screening. Ultrasound is often the first tool used because it’s quick, inexpensive, and radiation-free. It picks up simple cysts reliably, showing the characteristic round shape, smooth walls, and fluid-filled interior.
When a cyst looks more complex on ultrasound, doctors typically follow up with a contrast-enhanced CT or MRI to get a clearer picture. MRI tends to be more accurate than contrast-enhanced ultrasound for classifying complex cysts. In one prospective comparison, MRI matched CT findings with about 94% accuracy, while contrast-enhanced ultrasound agreed with CT only about 51% of the time. MRI also had much better specificity for ruling out concerning features. For straightforward simple cysts, though, a standard ultrasound is all that’s needed.
Monitoring and Follow-Up
If your cyst is classified as Bosniak I or II, current guidelines from urological associations recommend no follow-up imaging at all. These cysts are benign, and repeated scans add cost and anxiety without benefit.
Bosniak IIF cysts are a different story. Because they carry a small chance of progression, the recommended schedule is imaging every 6 to 12 months for the first year, then annually if the cyst remains stable. If nothing changes over five years of monitoring, the cyst can generally be considered benign, and surveillance can stop. If the cyst develops new thickening, nodules, or other worrisome features during that window, it gets reclassified and managed accordingly.
Treatment Options
The majority of simple renal cysts never need treatment. If a cyst grows large enough to cause pain or pressure symptoms, there are two main approaches.
The least invasive option is percutaneous aspiration with sclerotherapy. A radiologist inserts a needle through the skin under imaging guidance, drains the cyst fluid, and injects a hardening agent to collapse the cyst walls and prevent refilling. In one study of 66 cysts treated this way, 98.5% responded at one year. Nearly half disappeared completely, and another 36% shrank by more than 85%. Only a single cyst out of 66 showed no meaningful response.
For very large or recurrent cysts, or those in locations that make needle drainage difficult, a surgeon can remove the cyst wall laparoscopically. This involves a few small incisions and a camera-guided approach, and it carries a low recurrence rate. Recovery typically takes one to two weeks before returning to normal activities.
Bosniak III and IV cysts are handled differently because of their cancer risk. These usually require partial or complete removal of the affected kidney tissue, with the approach depending on the cyst’s size and location.
Renal Cysts vs. Polycystic Kidney Disease
A simple renal cyst is not the same thing as polycystic kidney disease (PKD). Simple cysts are acquired, meaning they develop over time as part of normal aging. They’re usually solitary or few in number and don’t impair kidney function. PKD is a genetic condition present from birth, causing hundreds or thousands of cysts to develop in both kidneys over a person’s lifetime. PKD progressively enlarges the kidneys and can lead to kidney failure, high blood pressure, and other complications. Having one or two simple cysts on an imaging scan does not mean you have PKD or are at risk for it.