Most lumps in the leg are benign. Benign soft tissue masses outnumber malignant ones by at least 10 to 1, so while finding a new lump can be alarming, the odds are strongly in your favor. The most likely explanations include lipomas, cysts, hematomas from an injury, or inflamed veins. That said, certain characteristics do warrant prompt evaluation, and knowing what to look for can help you decide how urgently to act.
Lipomas: The Most Common Cause
Lipomas account for roughly half of all benign soft tissue masses. They’re slow-growing deposits of fat that sit just beneath the skin, and they’re especially common in people between 40 and 70 years old or those carrying extra weight. A lipoma feels rubbery and doughy, not hard. It moves easily when you press on it, and it’s usually round or oval-shaped and symmetrical. Most are smaller than two inches across, though occasionally one can grow to six inches or more.
Lipomas in the leg tend to show up on the upper thigh, where they develop in the fatty tissue between skin and muscle. Deeper lipomas can sit within the muscle itself, and those ones move when you flex. They’re almost always painless unless they press on a nearby nerve. A lipoma doesn’t typically need treatment unless it’s painful, growing, or cosmetically bothersome, in which case surgical removal is straightforward.
Cysts That Form in the Leg
Several types of cysts can cause a noticeable lump. Epidermal inclusion cysts (sometimes still called sebaceous cysts) are dome-shaped bumps that develop around hair follicles. They’re mobile under the skin, range from a few millimeters to about 5 cm, and often have a small dark dot (called a punctum) at the center. They contain a buildup of keratin, the same protein in your hair and nails, and they can become red and tender if they get inflamed or infected.
Baker’s cysts form specifically behind the knee. These are fluid-filled pouches that develop when excess joint fluid pushes into the space at the back of the knee, often as a result of arthritis or a cartilage tear. They can feel like a water balloon and may cause stiffness or aching that worsens when you fully extend or bend the knee. If a Baker’s cyst ruptures, the fluid drains down into the calf, causing sudden swelling and pain that can mimic a blood clot.
Ganglion cysts are thick, jelly-like lumps that grow near joints, ligaments, or tendons. In the leg, they most commonly appear around the ankle or foot. They can cause pain, swelling, and joint stiffness, and they sometimes change size depending on your activity level.
Lumps From Injury or Exercise
A direct blow to the leg can cause a hematoma, which is a pocket of blood that collects under the skin or within the muscle. Initially it may feel soft and swollen, but once the blood stops flowing it firms up into a noticeable lump or clot. Over the following weeks, the hardened area gradually softens and the body reabsorbs the trapped blood. Every hematoma heals at its own pace, but the process is typically gradual, taking several weeks. If you remember banging your leg or taking a hard hit, a hematoma is the likely explanation for a firm, tender lump that appeared shortly afterward.
Muscle hernias are another exercise-related possibility. These happen when a small portion of muscle tissue pushes through a tear in the thin tissue (fascia) that wraps around it. The defining feature is that the lump gets bigger when you flex the muscle or stand up, and shrinks or disappears when you relax or lie down. They’re most common in the lower leg, particularly in runners and athletes. Muscle hernias are usually harmless, though they can occasionally cause aching or cramping.
Lumps on the Foot
Plantar fibromas are firm, nodular lumps that grow within the thick band of tissue on the sole of your foot. They’re the second most common soft tissue mass in the foot and ankle, after ganglion cysts. You’ll typically notice a hard bump in the arch that hurts more with walking or standing. They’re bilateral (appearing in both feet) in 20 to 50 percent of cases, so if you have one on each side, that actually supports the diagnosis rather than raising concern.
When a Lump Could Be a Blood Clot
Not every leg lump is a soft tissue mass. Superficial thrombophlebitis, an inflamed vein near the skin’s surface, creates a red, hard cord you can feel just under the skin. The area around it will be warm, tender, swollen, and visibly red. This is different from a round, mobile lump. It feels more like a firm line or rope running along the path of a vein.
Deep vein thrombosis (DVT) doesn’t usually present as a lump you can feel with your fingers. Instead, the whole leg or a large section of it becomes swollen, painful, and tender. If your “lump” is really generalized swelling in one leg, especially after a long flight, surgery, or a period of immobility, that pattern is more consistent with a DVT and needs urgent medical evaluation.
Warning Signs That Need Attention
UK clinical guidelines recommend that any soft tissue lump be considered potentially malignant if it’s larger than 5 cm (about 2 inches), located deep beneath the muscle layer, painful, and growing. Of these, increasing size is the single best individual indicator of higher risk. A mass that grows steadily over weeks to months is more concerning than one that appeared suddenly and painfully over a few days (which is more likely inflammatory or injury-related).
Here’s a practical checklist of features that should prompt you to get the lump examined sooner rather than later:
- Size over 5 cm (roughly the size of a golf ball or larger)
- Steady growth over weeks or months
- Deep location, meaning it feels fixed or attached to deeper structures rather than sliding freely under the skin
- Hard or firm texture that doesn’t feel rubbery or doughy
- Pain that isn’t explained by a recent injury
A lump that’s small, soft, mobile, and hasn’t changed in months is very unlikely to be anything serious. But any lump that checks multiple boxes on the list above warrants imaging.
How Leg Lumps Are Diagnosed
Ultrasound is typically the first imaging test for a leg lump. It’s inexpensive, widely available, and excellent at distinguishing solid masses from fluid-filled cysts. It also shows blood flow patterns within a mass, which helps differentiate benign from suspicious growths. For lumps that are large or located deep in the muscle, MRI is the preferred tool because of its superior ability to show the full extent of the mass and its relationship to surrounding tissues.
When researchers evaluated both imaging methods together, the combination achieved 91% sensitivity and 93% accuracy for identifying malignant versus benign masses. The key features doctors look for on imaging are the margin (whether the edges are smooth or irregular), the maximum diameter, and the blood vessel density within the lump. In many cases, imaging alone is enough to confirm a diagnosis like a lipoma or cyst without any need for a biopsy.