Swelling in the buttocks can come from a surprisingly wide range of causes, from something as simple as sitting too long on a hard chair to infections, cysts, or soft tissue injuries. The location, size, and accompanying symptoms (pain, redness, warmth, fever) are the best clues to narrowing down what’s going on. Here’s a breakdown of the most common causes and what distinguishes each one.
Pilonidal Cysts
One of the most frequent causes of a noticeable lump near the top of the buttocks is a pilonidal cyst, an unusual pocket in the skin that fills with hair and skin debris. These form almost exclusively near the tailbone, right at the top of the crease between the buttocks. The cause is typically loose hairs that puncture the skin, driven inward by friction from tight clothing, prolonged sitting, or activities like cycling. The body tries to wall off the intruding hair by forming a cyst around it.
Many pilonidal cysts cause no symptoms at all until they become infected. When infection sets in, the area becomes painful, red, and visibly swollen, sometimes draining pus or blood through a small opening in the skin. The drainage often has a noticeable odor. Pilonidal disease has become more common over time. A Danish population study tracking cases from 1996 to 2021 found the incidence rose from about 26 to nearly 40 per 100,000 people per year. It’s most common in young adults, particularly those with coarse or abundant body hair.
Ischial Bursitis (Sit Bone Pain)
If the swelling is deeper, centered on the bony point you sit on, ischial bursitis is a likely culprit. A bursa is a small fluid-filled sac that cushions the bone, and when it becomes inflamed, it can swell enough to form a soft, slightly tender mass in the lower buttock. This condition was historically called “weaver’s bottom” or “tailor’s bottom” because it’s caused by prolonged pressure on hard surfaces.
The hallmark symptom is pain in the buttock or upper back of the thigh that worsens after sitting for long periods or after exercise. You may also notice difficulty getting up from a chair, trouble climbing stairs, or reduced hip mobility. The swelling tends to be most noticeable when the hip is flexed. People in professions involving heavy vibration, such as tractor drivers or road equipment operators, are at higher risk. Athletes who repeatedly land on their backside, like horseback riders and canoeists, are also prone to it. Avoiding hard seating surfaces and taking breaks from prolonged sitting are the most effective ways to keep it from recurring.
Abscesses and Skin Infections
A gluteal abscess is a collection of pus that forms in the tissue of the buttock, usually from a bacterial infection. These present as a painful, warm, swollen area with red skin overlying it. One case report in the medical literature describes a typical presentation: a 48-year-old woman with a painful buttock swelling 10 centimeters across, with redness, localized heat, and a mild fever of about 100.6°F.
Abscesses can develop from skin bacteria entering through a small cut or hair follicle, from an infected cyst that has ruptured, or as a complication of conditions like Crohn’s disease, where inflammation in the bowel wall can create fistulas (abnormal tunnels) that track into the surrounding tissue. Cellulitis, a spreading skin infection without a defined pocket of pus, can also cause diffuse swelling and redness across a larger area of the buttock. Both conditions typically need medical treatment to resolve.
Injection Site Reactions
The buttock is one of the most common sites for intramuscular injections, and swelling after a shot in the gluteal area is not unusual. In most cases, it’s a mild, temporary inflammatory response that resolves on its own. But complications can be more serious. The traditional upper-outer buttock injection site carries risks including irritation of the fat layer under the skin, injury to branches of the gluteal artery, and in rare cases, damage to the sciatic nerve.
When an injection damages a small blood vessel, a hematoma (a pool of trapped blood) can form deep in the muscle. If that hematoma becomes infected, typically with Staphylococcus aureus bacteria, it can progress to a deep abscess. The warning signs of a problematic injection reaction include swelling that grows rather than shrinks over days, increasing pain, warmth, redness spreading outward from the injection site, or fever.
Trauma-Related Swelling
A direct blow or fall onto the buttocks can cause several types of swelling. The simplest is a bruise with associated soft tissue swelling that resolves on its own. More significant trauma, such as a car accident or a high-energy fall, can cause a Morel-Lavallée lesion. This happens when a shearing force separates the deep tissue layers from the skin and superficial fat, creating a pocket that fills with blood and lymph fluid. In the acute phase, you’ll notice pain, bruising, and soft tissue swelling. The skin over the area may feel unusually loose or “squishy” when pressed.
Repeated trauma to the buttock muscles can also lead to a condition where bone tissue forms within the muscle itself. This is the body’s misguided healing response, converting soft tissue into hard, calcified material at the injury site. It’s most common in people who experience repeated impacts to the same area.
Lipomas and Other Growths
A slow-growing, painless lump in the buttock is often a lipoma, a benign mass of fat cells. Lipomas feel soft and rubbery, usually sit relatively close to the skin surface, and rarely grow larger than about 2 inches across. They’re harmless and generally don’t need treatment unless they’re bothersome.
Rarely, a deeper or firmer mass could be a liposarcoma, a cancerous growth in the fat tissue. The key differences: liposarcomas tend to feel firm rather than soft, develop deeper in the tissue rather than just under the skin, and can grow substantially larger than a typical lipoma. Neither type usually causes pain, which is why size, depth, and firmness matter more than how it feels. Any lump that is firm, deep, or growing steadily deserves imaging. Ultrasound and MRI are both used to evaluate masses in the gluteal region, with MRI providing the most detailed look at deeper structures.
Inflammatory and Systemic Causes
Crohn’s disease can cause swelling in the buttock area when the inflammation extends beyond the bowel. Because Crohn’s affects all layers of the intestinal wall, it can create fistulas and abscesses that track into the tissue surrounding the anus and buttocks. This type of swelling is typically accompanied by other gastrointestinal symptoms and tends to recur.
Epidermoid cysts, which are slow-growing sacs filled with keratin (the protein that makes up skin and hair), can also form in the gluteal region. They’re usually painless and unnoticeable until they rupture, at which point they trigger an intense inflammatory reaction with sudden pain, swelling, and redness. Acne and prior skin trauma increase the likelihood of developing these cysts.
Signs That Need Prompt Attention
Most buttock swelling from minor causes resolves with time, rest, and avoiding the aggravating activity. But certain features warrant a faster evaluation: swelling that is hot to the touch and spreading, any fever alongside the swelling, a mass that is firm and growing, pus drainage, or swelling on one side accompanied by leg pain that won’t resolve (which can signal a deep vein blood clot). Numbness in the area between your legs, or sudden changes in bowel or bladder control alongside buttock swelling, are rare but serious signs that need immediate medical attention.