Hard bumps on the subcutaneous tissue of the arm may be caused by diseases such as dermatofibroma, boils, subcutaneous lipomas, and sebaceous cysts. 1. Dermatofibroma: the etiology may be related to trauma, insect bites and other factors. They mainly appear as round firm nodules, usually without conscious symptoms, and can be found in the armpits. 2. Boils: mainly related to Staphylococcus aureus infection, long-term alcohol consumption and poor hygiene habits can also induce the disease. Red nodules will appear at the beginning of the disease, and with the development of the disease, semi-circular solid nodules will appear, accompanied by pain, swelling, etc., which can occur in any part of the body. 3. subcutaneous lipoma: the arm is rich in fatty tissue under the skin, if you feel the small hard lumps may be lipoma, small lipomas are asymptomatic and can be observed and dealt with. 4. sebaceous cysts: if the sebaceous glands under the arm skin are blocked, it may trigger an inflammatory reaction, which in turn forms cysts with small hard bumps when touched, and if it is more obvious, surgical excision can be considered. There are more reasons for hard bumps on the subcutaneous tissues of the arm, you should go to the regular hospital for examination to clarify the cause of the disease.