Most bumps under the skin are lipomas or cysts, both of which are benign and extremely common. But several other conditions, from infections to autoimmune diseases to rare cancers, can also produce subcutaneous lumps. The specific characteristics of the bump, including its size, texture, location, and how fast it’s growing, help narrow down the cause.
Lipomas: The Most Common Cause
Lipomas are soft, rubbery lumps made entirely of fat cells that grow just beneath the skin. They affect roughly 1% of the population, making them far and away the most frequent explanation for a painless bump under the skin. A lipoma moves easily when you press on it, feels doughy rather than firm, and typically causes no pain at all. They can appear anywhere on the body but tend to show up on the neck, shoulders, back, abdomen, arms, and thighs.
Most lipomas stay small, under 5 centimeters, and grow very slowly over months or years. They don’t become cancerous. Many people have more than one. No treatment is necessary unless a lipoma is bothersome, painful, or growing. When removal is desired, a simple surgical excision usually takes care of it permanently.
Epidermoid Cysts
Epidermoid cysts are the other extremely common cause of under-skin bumps, and they’re frequently mistaken for lipomas. These are small, round sacs filled with a thick, cheese-like material made of keratin (the same protein in your hair and nails). They typically range from half a centimeter to several centimeters across and feel firm but slightly compressible when squeezed.
The key difference from a lipoma is that an epidermoid cyst often has a tiny dark dot at the center, called a punctum, which is essentially a blocked pore opening. Cysts are usually painless unless they rupture under the skin, at which point they can become red, swollen, and tender, closely resembling a boil. A ruptured cyst sometimes needs drainage or antibiotics, but an intact cyst that isn’t causing problems can be left alone.
Abscesses and Boils
If the bump under your skin is painful, warm, red, and swollen, an infection is the likely culprit. A boil (furuncle) starts as a tender lump around a hair follicle and gradually fills with pus. An abscess is a larger pocket of pus that can form deeper under the skin. Both are typically caused by bacteria entering through a cut, scratch, or ingrown hair.
Unlike lipomas and cysts, infected bumps tend to develop quickly over days rather than weeks. They may be accompanied by fever and chills. Smaller boils often drain and heal on their own, but larger abscesses generally need to be drained by a healthcare provider. Increasing redness, spreading warmth, or fever are signs the infection may be worsening.
Swollen Lymph Nodes
Lymph nodes are small, bean-shaped glands scattered throughout your body, with clusters in the neck, armpits, and groin. When your immune system is fighting an infection, nearby lymph nodes swell and can feel like firm, tender bumps under the skin. A cold, ear infection, or skin wound can all trigger this response.
Swollen lymph nodes from a common infection typically shrink back to normal within two to four weeks as the illness resolves. Lymph nodes that remain enlarged for longer, are hard and fixed in place, or keep growing without an obvious infection deserve medical evaluation, since persistent lymph node enlargement can occasionally signal lymphoma or metastatic cancer.
Dermatofibromas
Dermatofibromas are small, hard bumps that usually appear on the lower legs. They form from an overgrowth of fibrous tissue, sometimes triggered by a minor injury like an insect bite or a small cut. They’re typically less than a centimeter across, brownish or reddish in color, and feel like a firm pebble under the skin. Pinching one between your fingers often causes the center to dimple inward. These are harmless and rarely need treatment.
Conditions Linked to High Cholesterol
Certain types of under-skin bumps are actually deposits of cholesterol and fat, called xanthomas, that signal a lipid disorder. These bumps vary in appearance depending on type and location. Tuberous xanthomas are firm, painless, red-yellow nodules that develop over pressure points like the knees, elbows, and heels. They’re often linked to inherited lipid disorders. Yellowish bumps in the creases of your palms or between your fingers and toes can also indicate familial high cholesterol.
The most recognizable form is xanthelasma: soft, yellow or yellow-orange patches that appear around the inner corners of the eyelids. These start small and enlarge slowly over several months. Interestingly, xanthelasma can occur even when cholesterol levels are normal, though it’s associated with familial hypercholesterolemia and certain liver conditions. If you notice yellow-tinted bumps in any of these locations, lipid testing is worthwhile.
Rheumatoid Nodules
People with rheumatoid arthritis sometimes develop firm, rounded lumps under the skin called rheumatoid nodules. These typically appear near joints that are under pressure, especially the elbows, fingers, and heels. They range from pea-sized to several centimeters and are usually painless, though they can be tender if pressed against a nerve or joint. Rheumatoid nodules tend to develop in people with more severe or long-standing disease and are considered a sign of active inflammation.
Ganglion Cysts
Ganglion cysts are jelly-filled lumps that form along tendons or joints, most commonly on the wrist or hand. They feel firm and smooth, and their size can fluctuate, sometimes shrinking and then growing back. They’re not cancerous, but they can cause discomfort if they press on a nearby nerve. Ganglion cysts are especially common in women between 20 and 40 years old.
Soft Tissue Sarcoma
Rarely, a lump under the skin turns out to be a soft tissue sarcoma, a type of cancer that develops in fat, muscle, nerves, or connective tissue. These account for a very small fraction of all under-skin bumps, but they’re the reason any new or changing lump deserves attention. Sarcomas tend to grow steadily, may feel firm or fixed to deeper tissue (meaning they don’t slide around easily), and can eventually become painful as they press on surrounding structures.
Red flags that distinguish a potentially serious lump from a benign one include rapid growth over weeks, a size exceeding 5 centimeters (roughly the size of a golf ball), firmness or hardness, and being anchored in place rather than mobile. A lump with any of these features warrants prompt evaluation.
How Doctors Evaluate a Lump
A physical exam is usually the first step. Your doctor will assess the bump’s size, texture, mobility, and whether it’s painful. Many lipomas and cysts can be diagnosed on exam alone. When the cause isn’t clear, ultrasound is the go-to imaging tool for superficial lumps. It’s quick, painless, and effective at distinguishing solid masses from fluid-filled cysts. MRI or CT scans are reserved for deeper or larger lumps, or when there’s concern about a possible malignancy.
If imaging raises questions, a fine needle aspiration biopsy may follow. This involves inserting a thin needle into the lump to extract cells for examination under a microscope. It’s a quick outpatient procedure that can often provide a definitive answer. For lumps that test positive for abnormal cells, surgical removal and full tissue analysis confirm the diagnosis.
Features Worth Paying Attention To
Most under-skin bumps are harmless, and many people live with lipomas or cysts for years without issue. But certain characteristics should prompt you to get a lump evaluated:
- Rapid growth: a lump that noticeably increases in size over weeks
- Size over 5 centimeters
- Firmness or hardness, especially if the lump doesn’t move when pushed
- Pain that worsens or seems disproportionate to the size of the bump
- Skin changes: redness, bleeding, crusting, or ulceration over the lump
- Recurrence in the same spot after removal
A lump that has been the same size for years, moves freely, and causes no symptoms is very likely benign. A new lump that’s changing, growing, or causing pain is worth getting checked, even if the odds favor a harmless explanation.