A hard lump under the breast may be related to sebaceous cysts, breast hyperplasia, breast fibroma and so on. 1. Sebaceous cyst: due to the secretion of sebaceous glands in the skin under the breast is blocked in the pores, forming retention leading to cysts. It is usually a lump above the skin surface with a smooth surface. 2. Breast Hyperplasia: It may be related to heredity, environment, endocrine disorders, etc., causing cystic dilatation of breast ducts and formation of cysts of different sizes. 3. Breast fibroma: commonly found in young women, due to abnormal hormone levels in the body or hereditary reasons. The connective tissue around the glandular ducts proliferates, presenting a swelling with tough texture, smooth surface, and generally no obvious pain when pressed. At the same time, a hard lump under the breast may also be breast cancer, inflammatory lumps, breast duct papilloma, lipoma and other diseases. When there is a hard lump under the breast, it is recommended to consult a doctor in time, follow the doctor’s instructions for further examination, clarify the cause with the help of the doctor, and carry out targeted treatment or therapy.