A round hard lump inside the armpit may be a nodule, cyst, tumor, mostly considered to be boils, epidermal cysts, lipoma and other diseases. 1. Boil: Boils are mostly caused by Staphylococcus aureus infection, excessive sweating, scratching the skin, poor hygiene, alcoholism and other factors. Patients may have one or more painful nodules, which may appear as a round hard lump inside the armpit. 2. Epidermoid cyst: Epidermoid cyst is a kind of cyst containing keratin in the dermis, which may be caused by embryonic ectopic, skin trauma and other factors. Patients may have isolated intradermal nodules in the armpit, medium hardness, movable, with the development of the disease may be accompanied by pain, tenderness and other symptoms, some patients may be manifested as a round hard lump in the armpit. 3. Lipoma: the etiology of lipoma is not yet clear, it may be caused by genetic, chronic inflammation, systemic lipid metabolism abnormality and other factors. It is manifested as localized lumps, and patients may have a round hard lump inside the armpit. There are many reasons that lead to a round hard lump in the armpit, patients can go to the hospital in time to find out the cause of the disease to be treated accordingly.