Large lumps under the skin on the outer side of the big arm are usually referred to as subcutaneous lumps on the upper arm, which may be due to lipomas, epidermoid cysts, boils and other causes. 1. Lipoma: Lipoma is a tumor of normal fat-like tissue, preferably occurring in the limbs and trunk. They are well defined, lobulated, soft, pseudocystic and painless. The growth is slow, but it can be huge in size. The deep ones can be malignant, and should be removed in time. 2. Epidermoid cysts: Epidermoid cysts originate from the funnel part of the hair follicle, and the lesion is located in the dermis. The wall of the cyst is intact, the inner wall is smooth, and there is grayish white material in the cyst cavity. Tumors are mostly round, varying in size, adherent to the skin, and active at the base. When complicated with infection, it can be rapidly enlarged, red, swollen and painful, and can be ulcerated in serious cases. 3. Boils: can be caused by Staphylococcus aureus infection, can be single or multiple, usually soybean-sized or larger pimples, nodules. There may be other reasons for subcutaneous lumps on the upper arm, so patients are advised to go to the hospital in time to avoid delay.