The mass on the inner side of right upper arm may be caused by neurofibroma, and it may also be caused by lipoma or subcutaneous cyst. 1. Neurofibroma: Neurofibroma will lead to oval or round swelling on the skin surface or under the skin, usually this kind of swelling is hard, and can be shifted when pressed, in addition, patients with neurofibroma will also have symptoms such as coffee spots, and the mass on the inner side of the right upper arm can be considered to be caused by neurofibroma. 2. Lipoma: Lipoma is a kind of benign soft tissue tumor which is relatively common in clinic and usually produces single or multiple lumps under the skin locally. This disease is usually caused by long-term inflammation in the affected area or the body’s ability to metabolize fat abnormally. 3. Subcutaneous cysts: Subcutaneous cysts occur in the deeper layers of the skin and can form localized swellings in the patient’s skin, which are obviously painful to touch, and as they increase in size, they can lead to redness, swelling, ulceration and infection of the skin. A mass on the inside of the right upper arm may also be a subcutaneous cyst. The right inner arm lump, we should promptly go to the hospital for examination, under the guidance of the doctor to clarify the cause of the disease and targeted treatment, do not be negligent, so as not to delay the condition.