Most cysts appear as round, dome-shaped bumps just beneath the skin, typically flesh-colored or slightly yellowish, and they range from smaller than a pea to several centimeters across. They’re usually smooth, slow-growing, and painless. But “cyst” is a broad term, and the exact appearance depends on where it forms and what type it is. Here’s how to identify the most common kinds.
The Typical Skin Cyst
The most common type you’ll encounter is an epidermoid cyst (sometimes incorrectly called a sebaceous cyst). These are firm, round nodules that sit just under the skin and range from flesh-colored to yellowish. They move slightly when you press on them, and they grow slowly over weeks or months.
The signature feature is a tiny dark dot at the center, sometimes called a punctum. This looks like a small blackhead plugging a central opening. Not every cyst has one, but when it’s there, it’s a strong visual clue. If the cyst ruptures or gets squeezed, it can release a thick, cheesy, strong-smelling substance called keratin. This is normal cyst material, not pus, though the smell can be alarming.
Scalp Cysts Look Different
Cysts on the scalp are usually a different type called pilar (or trichilemmal) cysts. About 90% of pilar cysts show up on the scalp. They feel firm and mobile under the skin, ranging from 0.5 to 5 cm across. The key visual difference from a standard epidermoid cyst: pilar cysts have no central punctum. The surface is smooth and unbroken. Their walls are also thicker and less likely to rupture on their own, so they tend to stay intact longer.
Ganglion Cysts Near Joints
Ganglion cysts form around joints and tendons, most often on the wrist or hand. They look like a small, firm, rubbery bump sitting on top of the joint. The Mayo Clinic describes them as resembling “a tiny water balloon on a stalk.” They’re filled with thick, jelly-like fluid similar to what naturally lubricates your joints.
One distinctive feature: ganglion cysts change size. They often get larger with repetitive joint movement and may shrink when you rest the area. They can range from 2 to 4 cm in diameter and may be visible or only felt beneath the skin. If a doctor shines a light through them, the light passes through (a sign that the lump is fluid-filled rather than solid).
Baker’s Cysts Behind the Knee
A Baker’s cyst forms as a fluid-filled bulge behind the knee. You’ll notice swelling in the back of the knee joint, sometimes extending down into the calf. It creates a feeling of tightness, especially when you straighten or fully bend the leg. From the outside, it looks like a soft, rounded swelling in the hollow behind the knee. These are often associated with knee injuries or arthritis that cause excess fluid to build up in the joint.
Tiny White Bumps: Milia
Milia are miniature cysts, only 1 to 2 mm across, that appear as small white or pearly dome-shaped bumps on the skin surface. They’re extremely common on the face, especially around the eyes, nose, and cheeks. Unlike a pimple, milia feel hard to the touch and don’t have a red, inflamed base. They contain trapped keratin and often show up in clusters.
Eyelid Cysts vs. Styes
A chalazion is a cyst that forms on the eyelid when an oil gland gets blocked. It starts as a small, painless bump farther back on the eyelid, away from the lash line. At first you may barely notice it. As it grows, the eyelid can become red and mildly swollen, but a chalazion rarely makes the entire eyelid puff up.
A stye, by contrast, is not a cyst. It’s an infection at the base of an eyelash. Styes are very painful, appear right at the eyelid’s edge, and often feature a small pus spot at the center of a red, angry bump. Styes frequently cause the whole eyelid to swell. If your eyelid lump is deep, relatively painless, and away from the lash line, it’s more likely a chalazion.
How to Tell a Cyst From a Boil
Cysts and boils can look similar at first glance, but they feel quite different. Cysts are generally painless, slow-growing, and smooth. They sit under the skin as moveable lumps. Boils, on the other hand, are hard, painful, red or discolored lumps that form around hair follicles. They’re caused by infection and tend to be tender from the start.
Boils often begin as an itchy, sore spot and progress to show visible yellow pus at the surface. They range in size from a cherry pit to a walnut. Cysts don’t typically show pus unless they’ve become infected. A boil also develops faster, over days rather than weeks.
What an Infected Cyst Looks Like
A stable, healthy cyst is the same color as your surrounding skin (or slightly yellowish), painless, and unchanged from week to week. When a cyst becomes infected, the appearance shifts noticeably. The skin over and around the cyst turns red or darker than your normal skin tone. The area swells beyond the original size of the cyst, becomes warm to the touch, and starts to hurt. You may see fluid or pus draining from the surface.
An infected cyst can look a lot like a boil at this stage, which is part of why the two get confused. The key history is whether the lump was there before the pain started. A cyst that was sitting quietly for weeks and then suddenly becomes red and painful has likely become inflamed or infected.
When a Lump Needs Attention
Most cysts are completely harmless. But any lump that changes rapidly in size or shape, develops irregular borders, bleeds without being squeezed, or keeps coming back after draining deserves a closer look from a healthcare provider. The American Cancer Society notes that while the vast majority of cysts are benign, changes in size, shape, or symptoms are the signals worth acting on. A simple physical exam, and occasionally an ultrasound, is usually all it takes to confirm what you’re dealing with.