Most cysts appear as small, round bumps under the skin, often skin-colored or slightly yellowish, and they feel firm or slightly springy when pressed. They can range from barely visible (less than a sixteenth of an inch) to as large as four inches across, though most stay well under an inch. What a cyst looks like depends on its type and location, but a few visual clues are consistent across nearly all of them.
Skin Cysts: The Most Common Type
The cysts most people encounter are epidermoid cysts, sometimes still called sebaceous cysts. These show up as smooth, round lumps just beneath the skin surface, most often on the face, neck, chest, back, or trunk. The skin over the cyst usually looks normal, matching your skin tone, though it can appear slightly stretched or shiny if the cyst grows large.
One of the most distinctive features is a tiny dark dot at the center of the bump. This is a small pore or opening, sometimes described as a blackhead plugging the top of the cyst. Not every cyst has one, but when it’s there, it’s a strong visual clue that you’re looking at an epidermoid cyst rather than another type of lump. If squeezed or drained, these cysts release a thick, yellowish, cheese-like substance with a noticeably foul smell. That material is a buildup of the protein keratin and dead skin cells that accumulates inside the cyst’s sac.
Pilar Cysts on the Scalp
Pilar cysts grow almost exclusively on the scalp, and most people discover them while washing or combing their hair. They feel like smooth, firm, flesh-colored lumps that move slightly under your fingers. Unlike epidermoid cysts, pilar cysts typically lack that central dark pore and have a smoother surface. They can grow quite large over time, sometimes reaching the size of a golf ball, though they usually stay painless unless irritated by repeated friction from brushing or styling.
Ganglion Cysts Near Joints
Ganglion cysts look and feel different from skin cysts because they form near joints and tendons, most commonly on the wrist or hand. They appear as round or oval lumps that sit on top of a joint, sometimes described as looking like a small water balloon on a stalk. They can be firm or slightly squishy depending on how much fluid they contain. Ganglion cysts are filled with a thick, jelly-like fluid rather than the keratin paste found in skin cysts. Their size can fluctuate, growing larger with repetitive joint use and sometimes shrinking on their own.
Baker’s Cysts Behind the Knee
A Baker’s cyst creates a noticeable fluid-filled bump on the back of the knee. It often looks like a soft, egg-shaped swelling in the crease behind the joint, and it may become more visible when you stand or fully extend your leg. Swelling can extend into the thigh or calf, and if the cyst ruptures, it can cause more dramatic swelling and discoloration in the lower leg that sometimes mimics a blood clot. Baker’s cysts typically develop alongside other knee problems like arthritis or cartilage tears, so the swelling rarely appears in isolation.
How a Cyst Differs From Other Lumps
Cysts and lipomas (fatty lumps) can look almost identical from the outside, which is why many people have trouble telling them apart. A few differences help:
- Texture: Cysts tend to feel firm and defined, like a marble or pea under the skin. Lipomas feel soft and doughy, more like a piece of rubber.
- Movement: Both move when pressed, but lipomas slide around more freely. Cysts may feel slightly more anchored to the skin above them, especially if they have a visible central pore.
- Contents: If a lump ever drains or leaks a thick, smelly substance, it’s almost certainly a cyst. Lipomas are solid fat and don’t produce discharge.
In practice, even doctors sometimes need imaging to confirm which one they’re dealing with.
What an Infected or Inflamed Cyst Looks Like
A cyst that becomes inflamed turns red, swollen, and tender to the touch. The skin over it may feel warm, and the bump can double or triple in size over just a few days. An infected cyst may develop a visible white or yellow head, similar to a large pimple, and can begin leaking pus or that characteristic thick, foul-smelling material on its own. Pain is the biggest change. Most cysts are painless until they become irritated by pressure, friction, or bacteria entering through the central pore.
Inflammation doesn’t always mean infection. Cysts can become irritated from being squeezed, bumped, or rubbed by clothing. The redness and swelling look similar either way, but true infection typically brings increasing pain, spreading redness beyond the borders of the cyst, and sometimes fever.
When a Lump Needs Closer Evaluation
Most cysts are completely benign. Cysts that appear uniform on ultrasound or imaging are almost always harmless and simply monitored over time. However, a few features warrant a closer look: rapid growth over weeks rather than months, an irregular shape or borders that aren’t smooth and round, hardness that feels fixed to deeper tissue rather than movable, or a cyst that keeps returning in the same spot after being drained. Cysts with solid components inside, rather than purely fluid, sometimes need further evaluation with repeat imaging to track any changes in size or structure.