A cyst is a closed, sac-like pocket of tissue that fills with fluid, air, or semi-solid material like keratin or oil. Cysts can form almost anywhere in the body, from just beneath the skin to deep inside organs like the kidneys, ovaries, or brain. The vast majority are benign and never become cancerous, though some need treatment if they grow large, become infected, or cause pain.
How Cysts Form
Cysts develop for a variety of reasons, and the cause often depends on where they appear. On the skin, a cyst typically forms when a gland or duct gets blocked. Skin cells that would normally shed to the surface instead get trapped beneath it, creating a small pocket that fills with oily secretions or protein. This is how the most common skin cyst, the epidermal inclusion cyst (often called a sebaceous cyst), develops.
Inside the body, the mechanisms are different. Ovarian cysts usually form as part of the normal menstrual cycle, when a follicle that releases an egg doesn’t open or close properly. Kidney cysts can result from genetic mutations that cause the lining of tiny tubes in the kidneys to grow abnormally and secrete excess fluid. Joint-related cysts, like the Baker’s cyst behind the knee, form when an underlying problem like arthritis or a cartilage tear causes the joint to produce too much lubricating fluid, which pools into a sac behind the joint.
Chronic inflammation, infections, inherited conditions, and simple blockages can all trigger cyst formation. In some cases, no clear cause is ever identified.
Common Types
Dozens of cyst types exist, but a handful account for most of what people encounter:
- Epidermal inclusion cyst (sebaceous cyst): A slow-growing bump under the skin, most often on the face, neck, or torso. It contains a thick, cheese-like material made of keratin.
- Ganglion cyst: A firm, rubbery lump near a joint or tendon, most commonly on the wrist or hand. It’s filled with a jelly-like fluid.
- Ovarian cyst: A fluid-filled sac on or inside an ovary. Extremely common. A large screening study found that 14% of women over 55 had a simple ovarian cyst the first time their ovaries were examined by ultrasound, and about 8% developed a new one each year.
- Baker’s cyst: A fluid-filled swelling behind the knee, usually linked to arthritis or a cartilage tear that causes the joint to overproduce lubricating fluid.
- Breast cyst: A round, fluid-filled sac inside the breast. These are especially common in women in their 40s and 50s and are almost always benign.
- Pilonidal cyst: Forms near the tailbone, often containing hair and skin debris. These are prone to infection.
- Kidney cyst: Simple kidney cysts become more common with age and rarely cause symptoms. Complex kidney cysts sometimes need closer monitoring.
What a Cyst Feels Like
Many cysts cause no symptoms at all. Internal cysts in the kidneys, brain, or ovaries are often discovered by accident during imaging for something else entirely. When a cyst does produce symptoms, the experience depends on its location and size. A skin cyst usually feels like a smooth, round bump that moves slightly under your finger. It’s typically painless unless it becomes inflamed or infected.
A Baker’s cyst behind the knee can create a feeling of tightness or fullness, especially when you straighten the leg. Ovarian cysts may cause a dull ache on one side of the lower abdomen, bloating, or pressure. Larger cysts anywhere in the body are more likely to press on surrounding tissue and cause discomfort.
Cysts vs. Tumors
People often worry that a new lump might be cancer, and this is one of the most important distinctions to understand. A cyst is primarily a fluid-filled sac. A tumor is an abnormal growth of solid tissue. They can feel similar from the outside, and imaging is usually needed to tell them apart.
On ultrasound or CT, a cyst that appears uniform and completely fluid-filled is almost always benign and simply needs to be monitored. If a cyst contains solid components mixed with fluid, it requires further evaluation because the solid portions could be benign or, less commonly, malignant. A biopsy, where a small sample of tissue is examined under a microscope, is the definitive way to determine whether any growth is cancerous.
How Cysts Are Diagnosed
If you notice a lump under your skin, a healthcare provider can often identify it as a cyst through a physical exam alone. For deeper or less obvious growths, ultrasound is the standard first step. It’s quick, painless, and excellent at distinguishing fluid-filled cysts from solid masses near the surface. For cysts in more complex areas of the body, CT or MRI scans provide a more detailed picture.
Biopsy is not a first-line step. Imaging alone can confidently diagnose roughly 20% to 25% of soft tissue masses, many of which turn out to be benign and never need a biopsy at all. The general approach is to fully characterize any mass with imaging before considering tissue sampling.
Treatment Options
Most cysts don’t require treatment. Simple ovarian cysts, for example, frequently resolve on their own. In the screening study mentioned above, 32% of ovarian cysts had disappeared entirely within a year without any intervention.
Treatment becomes necessary when a cyst hurts, gets infected, interferes with movement or daily activities, or is cosmetically bothersome. The two main approaches are drainage and surgical removal:
Drainage involves numbing the area with a local anesthetic, making a small incision, and letting the fluid or material inside escape. It provides fast relief, but there’s a catch: because the sac wall remains in place, the cyst often refills and comes back.
Surgical excision removes the entire cyst, including the surrounding sac lining, through a slightly larger incision. This takes a bit longer and requires stitches, but it significantly lowers the chance of recurrence. Both procedures are typically done with local anesthesia, meaning you’re awake but the area is numb.
Signs of an Infected or Ruptured Cyst
An uncomplicated cyst is usually painless and skin-colored. If it becomes infected, you’ll notice redness or discoloration of the overlying skin, swelling, warmth, and increasing pain. The area may feel tender to the touch and could begin draining pus.
A ruptured cyst, where the sac wall breaks open, causes similar symptoms: sudden pain, swelling, skin discoloration, and often a foul-smelling yellowish discharge. Rapid growth or a cyst that exceeds about 5 centimeters (roughly 2 inches) in diameter also warrants a closer look. These signs don’t necessarily mean something dangerous is happening, but they do indicate the cyst needs professional attention rather than a wait-and-see approach.