A cyst is a closed pocket or pouch of tissue that forms in your body, filled with fluid, air, pus, or semi-solid material. Cysts are extremely common, mostly harmless, and can develop nearly anywhere, from just beneath the skin to deep inside organs like the kidneys, liver, and ovaries. Most people will have at least one at some point in their lives, and many never realize it.
How Cysts Form
Cysts develop when something disrupts the normal structure of tissue, creating a sealed sac that the body walls off. The most common triggers are blocked ducts or glands. When oil, mucus, or other fluids can’t drain normally, they build up inside a pocket lined with tissue cells. Infections can also lead to cysts when the body encapsulates bacteria or pus. Chronic inflammation, genetic conditions, injuries, and even parasites can set the process in motion.
What makes a cyst different from a random buildup of fluid is its structure: a true cyst has a distinct wall, or lining, that separates its contents from surrounding tissue. This wall is what makes cysts self-contained and, in most cases, slow-growing and stable.
Skin Cysts
The cysts most people notice first are the ones that appear as small, round bumps under the skin. The most common type is the epidermoid cyst, which forms when surface skin cells get pushed deeper into the dermis and start multiplying. These cells produce keratin, the same protein that makes up your hair and nails, and that keratin accumulates inside the cyst as a thick, white-yellow paste. You’ll often see a small dark dot (a punctum) at the center.
The term “sebaceous cyst” gets used interchangeably with epidermoid cyst, but it’s technically a misnomer. True epidermoid cysts don’t contain oil-producing glands. Another related type, the pilar cyst, appears almost exclusively on the scalp and tends to run in families. Both epidermoid and pilar cysts are benign. They grow slowly, rarely cause problems unless they become infected, and can often be left alone unless they’re bothersome.
Ovarian Cysts
Ovarian cysts are remarkably common in women of reproductive age and rare after menopause. The majority are functional cysts, meaning they form as a normal part of the menstrual cycle. Each month, the ovary produces a small fluid-filled sac (a follicle) to release an egg. Sometimes that follicle doesn’t open to release the egg, or it reseals after release and fills with fluid. These cysts typically resolve on their own within a few weeks.
In a review of 244 ovarian cyst cases, functional cysts accounted for about 33% of all types, followed by benign cyst-adenomas (19%) and dermoid cysts (12%). Dermoid cysts are an unusual type that can contain hair, teeth, or fatty tissue because they arise from the same cells that produce eggs, which have the potential to develop into many tissue types.
Most ovarian cysts cause no symptoms at all and are discovered incidentally during imaging for something else. When they do cause trouble, the symptoms tend to be a dull ache or pressure on one side of the lower abdomen, bloating, or pain during certain activities.
Kidney and Other Internal Cysts
Kidney cysts are among the most frequently found internal cysts, appearing in roughly 27% of patients who undergo imaging studies. The rate climbs steadily with age: up to 55% of people over 70 have them. In older adults, kidney cysts account for 65% to 70% of all kidney masses found on scans. The vast majority are simple cysts that require no treatment. Patients typically have no symptoms and a normal physical exam.
Liver cysts follow a similar pattern. They’re common, usually asymptomatic, and found incidentally. Pancreatic cysts are less common but worth knowing about because they form through a different mechanism: injury, inflammation, or leakage of digestive enzymes that damage surrounding pancreatic tissue. Very large cysts in any organ can eventually press on neighboring structures and cause discomfort or other symptoms, but this is uncommon.
Parasitic cysts, called hydatid cysts, deserve special mention. These develop from infection with a small tapeworm and typically form in the liver or lungs. They’re rare in most developed countries but common in parts of the world where people live in close contact with livestock.
How Cysts Differ From Tumors
The distinction between a cyst and a tumor comes down to structure. A simple, benign cyst appears on imaging as a thin-walled, fluid-filled sac with smooth borders, no internal blood flow, and no solid components. On ultrasound, it looks like a dark, well-defined circle. On CT or MRI, it has predictable, uniform density.
A solid tumor, by contrast, is made of densely packed cells rather than fluid. And a cystic tumor (a tumor with fluid-filled areas) will show warning signs: thick walls, solid nodular components, internal blood vessels, and irregular borders. Enhancing septations, which are internal dividers that light up with contrast dye because they contain blood vessels, can signal malignancy.
Most cysts are clearly benign on standard imaging. When a cyst has complex or suspicious features, your doctor may recommend a closer look with MRI or, in some cases, aspiration (draining a sample of the fluid with a needle) to examine the contents.
How Cysts Are Diagnosed
Many cysts near the skin surface can be identified by feel and appearance during a physical exam. For anything deeper, imaging is the primary tool. Ultrasound is usually the first step because it’s fast, widely available, and excellent at distinguishing fluid-filled structures from solid ones. A simple cyst on ultrasound appears as a smooth, thin-walled structure with no internal blood flow.
MRI is particularly useful when more detail is needed. It can reliably distinguish a solid mass from a cystic one and detect subtle features like tiny solid components, blood products, or signs of infection. CT scans also play a role, especially for cysts in the abdomen and kidneys. On CT, a simple cyst has very low density, thin walls, few or no internal dividers, and doesn’t change appearance when contrast dye is injected.
One important caveat: cysts that contain blood, protein-rich fluid, or debris from infection can mimic solid masses on CT. When there’s ambiguity, MRI helps clarify what’s actually there.
Treatment and When Cysts Need Attention
Most cysts need no treatment at all. Simple kidney cysts, small ovarian cysts, and uncomplicated skin cysts can be safely monitored. Many resolve on their own, particularly functional ovarian cysts. When treatment is necessary, it usually involves drainage or surgical removal, depending on the location and whether the cyst keeps coming back.
Skin cysts that become inflamed or infected may need to be drained and treated for the infection. Complete removal of the cyst wall reduces the chance of recurrence. For ovarian cysts that persist, grow large, or cause significant pain, minimally invasive surgery is the most common approach.
Signs of a Cyst Emergency
While most cysts are harmless, certain complications need prompt medical attention. A ruptured ovarian cyst can cause sudden, sharp pain in the lower belly or back, vaginal spotting or bleeding, and abdominal bloating. These symptoms alone may resolve without intervention, but specific warning signs raise the urgency: severe nausea and vomiting (which may indicate the ovary has twisted on itself, cutting off its blood supply), fever (suggesting infection), heavy vaginal bleeding, or faintness and dizziness (which can indicate internal bleeding).
Ovarian torsion is particularly time-sensitive. The weight of a large cyst can cause the ovary to rotate, reducing or cutting off blood flow entirely. This requires surgery to restore circulation and prevent permanent damage. If an infected cyst of any type ruptures, it can release bacteria into the bloodstream and trigger sepsis, a life-threatening immune response that requires emergency care.