Lump on Arm: Causes and When to Worry

What a lump on the arm usually means

Most arm lumps come from the skin or the fat layer just under it. These are easy to feel, move when you push them, and are almost always benign (not cancer). Lumps that start deeper, in muscle, tendon or bone, are less common and more often need a scan.

Doctors judge a lump in the arm mainly by four things:

• Size: under 5 cm (2 inches) is much less likely to be serious.
• Depth: a lump in the skin or fat is usually harmless. One under the muscle layer needs imaging.
• Feel: soft and rubbery is reassuring. Rock-hard or fixed to deeper tissue is not.
• Behavior over time: a lump unchanged for years is reassuring. One that grows over weeks to months is not.

Common causes of a lump on the arm

Lipoma

A lipoma is a slow-growing lump of fat cells. It feels soft, doughy and smooth, slips under your fingers, and is painless. The upper arm, shoulder and forearm are common sites. Lipomas are most often found in adults aged 40 to 60, and some people have several. They do not turn into cancer. Removal is optional unless the lump is bothersome or large.

Epidermoid or sebaceous cyst

A skin cyst is a round sac of keratin (skin protein) under the skin. It feels firm and smooth, moves with the skin, and often has a small dark opening (punctum) on top. If squeezed it can release a thick, cheesy, smelly material. Cysts can become inflamed or infected, turning red and painful. These are often called sebaceous cysts, although most are technically epidermoid cysts.

Dermatofibroma

A dermatofibroma is a small, hard, button-like bump, usually 5 to 10 mm across, pink to brown in color. It is common on the arms and legs, often after a minor injury or insect bite. The giveaway is the "dimple sign": pinching the skin on either side makes the bump sink inward. It is harmless.

Insect bites and skin reactions

Insect bites, including mosquito, spider and bed bug bites, cause itchy, raised bumps that last a few days. A larger local reaction can produce a firm, swollen lump several centimeters wide that fades within a week.

Boils, abscesses and infected hair follicles

A boil is an infected hair follicle. An abscess is a larger pocket of pus. Both are red, warm, painful and get bigger over a few days, sometimes coming to a yellow head. Small boils often drain on their own with warm compresses. Larger abscesses need to be drained by a clinician.

Bruise, hematoma or muscle injury

A knock or fall can cause a hematoma, a collection of clotted blood that feels like a firm, tender lump under a bruise. It usually softens and shrinks over 2 to 4 weeks. Occasionally the blood hardens into bone inside the muscle (myositis ossificans), leaving a hard lump after a deep thigh or upper arm bruise.

Vaccine and injection reactions

A tender, firm lump at the site of a vaccine or injection in the upper arm is common and usually resolves in 1 to 2 weeks. A lump that grows, becomes hot and red, or pulsates should be checked.

Swollen lymph nodes

Lymph nodes sit at the inner elbow (epitrochlear nodes) and in the armpit. They swell in response to an infection in the hand or arm, such as an infected cut or cat scratch disease. Swollen lymph nodes that persist beyond 2 to 3 weeks, feel hard or matted, or come with fevers, night sweats or weight loss need assessment.

Ganglion cyst

A ganglion cyst is a fluid-filled lump arising from a joint or tendon sheath, most often on the back of the wrist. It can feel firm, change size, and may be painful when pressing on a nerve. It often goes away on its own.

Less common and serious causes

Sarcomas are rare: fewer than 1 in 100 soft tissue lumps checked by doctors turn out to be malignant. They matter because early diagnosis changes treatment, so lumps with the warning features above should not be watched indefinitely.

Lump on left arm vs right arm

The side of the body does not change the likely cause. A lump on the left arm has the same causes as one on the right. People sometimes worry that left arm symptoms mean a heart problem, but heart disease causes pain, pressure or heaviness in the arm, not a lump. A lump in the side of the arm or the upper inner arm is usually a lipoma, cyst or lymph node.

Fluid-filled lump on the arm

A soft lump that feels like it contains fluid is usually a cyst, a ganglion, a bursa (for example olecranon bursitis at the back of the elbow), a seroma after surgery, or an abscess if it is red and painful. An ultrasound can confirm that a lump is fluid-filled. Do not try to pop or drain a lump yourself, because this risks infection and scarring.

How doctors check a lump in the arm

Questions you may be asked

• How long have you had it, and has it grown?
• Is it painful, itchy or tender?
• Did it follow an injury, bite, injection or infection?
• Do you have fevers, night sweats, tiredness or weight loss?
• Have you had any skin cancer or other cancer before?

Examination

The doctor feels the size, texture and depth of the lump, whether it moves, and whether it is attached to skin or deeper tissue. They also check nearby lymph nodes in the armpit and elbow.

Ultrasound of an arm lump

Ultrasound is the usual first test. It is quick, painless and shows whether a lump is solid or fluid-filled, how deep it is, and whether it has a blood supply. Many lipomas and cysts can be confirmed this way. Under NICE guidance, an unexplained lump that is growing should have an urgent ultrasound, and a suspicious result leads to a sarcoma specialist referral.

Further tests

• MRI: for deep lumps, large lumps, or when ultrasound is unclear.
• Biopsy: a core needle biopsy takes tissue samples, ideally at a specialist center if sarcoma is possible.
• Blood tests: if infection or lymphoma is suspected.
• Skin biopsy or dermatoscopy: for skin-surface bumps that may be skin cancer.

Treatment for lumps on the arm

Treatment depends on the cause:

• Lipomas and cysts: no treatment is needed if they cause no trouble. They can be removed under local anesthetic in a short outpatient procedure. Cysts are best removed when not inflamed, with the whole sac taken out to stop them returning.
• Inflamed cysts and abscesses: warm compresses for small ones. Larger ones are cut and drained. Antibiotics are added if surrounding skin is infected or you are unwell.
• Bites: a cold compress, an oral antihistamine and a mild steroid cream ease itching.
• Hematomas: rest, ice for the first 48 hours, then gentle movement. Most resolve without treatment.
• Ganglion cysts: watching, aspiration with a needle, or surgical removal.
• Cancers: surgery, sometimes with radiotherapy or chemotherapy, planned by a specialist team.

Frequently asked questions

Should I be worried about a lump on my arm?

Most lumps on the arm are harmless lipomas, cysts or bite reactions. See a doctor if the lump is bigger than 5 cm (2 inches), hard, fixed, growing, deep in the muscle, painful without a reason, or comes with fever, night sweats or weight loss.

What does a cancerous lump on the arm feel like?

A soft tissue sarcoma typically feels firm, does not move easily, sits deep under the muscle, and keeps growing over weeks to months. It is often painless at first. Skin cancers look like a changing, crusted, bleeding or unusual pigmented bump. Only a scan and biopsy can confirm it.

What is a hard lump under the skin on my arm that doesn't move?

A small, hard bump fixed to the skin is often a dermatofibroma, which dimples when pinched. A hard lump fixed to deeper tissue or bone, especially one that is growing, needs an ultrasound or MRI to rule out a tumor.

Can a lump on the arm go away on its own?

Yes. Bite reactions, bruises, hematomas, vaccine-site lumps and small boils usually settle in days to weeks. Ganglion cysts sometimes disappear. Lipomas and epidermoid cysts usually stay unless removed.

Do I need an ultrasound for a lump on my arm?

Not always. A doctor can often diagnose a small, soft lipoma or a typical cyst by examination. An ultrasound is recommended when the lump is large, growing, deep, firm, or the diagnosis is unclear. It is quick, painless and uses no radiation.

Why do I have several lumps on my arms?

Multiple soft lumps are often lipomas, which can run in families. Multiple small tender lumps under the skin can be angiolipomas. Many soft bumps along with café-au-lait skin patches can suggest neurofibromatosis, which a doctor should assess.