A Hard Bump Under the Skin on My Arm

Your skin is your body's living armor. But this unsung hero often doesn't get much attention until something goes wrong -- like an unexplained bump under the skin on your arm. There is a lengthy list of possible causes, most of which are relatively minor conditions that will either disappear on their own or require minimal treatment. There are, however, some potentially serious conditions that should not be ignored. Characteristics of the bump, such as size, consistency and growth rate, can help point toward possible causes.

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Injuries and Bites

Injuries are common culprits for lumps and bumps on the arm. For example, a high-energy blow to the arm can cause not only a superficial bruise but deep tissue swelling. In some cases, there may be a bone fracture causing the bump. Puncture wounds and bug bites are other common causes of bumps on the arm. Lumps associated with bug bites vary in size, depending on what type of bite it is. The area is often pink or red, and may be itchy.

Minor puncture wounds, like a sliver or getting stuck with a thorn, can cause a similar bump -- especially if part of the object remains embedded in the skin. Bug bites and puncture wounds sometimes lead to an abscess, a pocket of infection. In this situation, the bump typically enlarges, becomes more tender, red and firm, and may ooze pus.


A bump on the arm unrelated to an injury or bite is often a cyst. Cysts are enclosed sacs usually filled with fluid or soft tissue. There are several types of cysts, which vary in size, have a firm consistency, tend to grow slowly, and are usually painless. They can occur anywhere in the body, though some types of cysts frequently develop in the arms. Ganglion cysts, for example, most commonly occur on the wrists and hands but can also develop near the elbow and shoulder. These fluid-filled cysts arise from the joint or nearby tendons. They are more common in women than men and can occur at any age.

An epidermoid cyst is another example of a cyst that can develop in the arm, though the face, trunk and back are more common sites. These dome-shaped cysts are more common in men than women, and arise from the upper skin layers. They sometimes have a blackish point toward the center, causing them to resemble a large blackhead. If the cyst drains through this opening, the inner material has a foul odor and a cheese-like consistency.

Noncancerous Tumors

Many types of cells and tissues are present in the arm, any of which can potentially give rise to a tumor. Fortunately, most of these tumors are not cancerous. Fibromas are a common example. These noncancerous tumors arise from fat cells and most often occur in people older than age 40. They typically grow very slowly and require no treatment.

Dermatofibromas are another relatively common example. These noncancerous tumors have a central area of scar tissue, which gives them a very firm consistency. Dermatofibromas often resemble a mole but a deeper lump can be felt beneath the skin surface. Other tissues can give rise to noncancerous tumors that might cause a bump in the arm, including muscle, bone, cartilage and others.

Cancerous Tumors

Several types of cancer can cause bumps in the arm. Skin cancer is most common, in part because the arms typically have a lot of sun exposure. While skin cancers characteristically have a superficial component, the tumor frequently extends deeper into the skin causing a raised lump.

Non-skin cancers can also affect the arms and cause bumps under the skin. For example, sarcomas are cancerous tumors that arise from tissues such as muscle, bone, tendon and fat. Although sarcomas can occur anywhere in the body, they most commonly develop in the arms, legs, abdomen or chest. Cancerous lymph nodes can also sometimes cause lumps in the arms.

Next Steps and Precautions

Most bumps under the skin on the arm do not indicate a serious problem. However, it's important not to ignore a lump that doesn't go away within a week to 10 days, or is accompanied by possible warning signs. See your doctor as soon as possible if you experience: -- spreading redness surrounding the lump -- severe or worsening pain -- numbness, tingling or paleness of the arm or hand -- growth of the lump -- changing or irregular color -- bleeding or itchiness in a mole-like lump -- pus-like discharge -- fever, chills or sweats -- unexplained weight loss

Reviewed by: Tina M. St. John, M.D.

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