Men with hard lumps on one side of the breast are considered to be related to breast development, breast hyperplasia, mastitis, breast cancer and so on. 1. Mammary gland development: when a variety of endocrine hormone imbalance in the male body, it may induce the development of the mammary gland, which may be manifested in the phenomenon of hard lumps on one side of the mammary gland and not on the other side. 2. Mammary hyperplasia: mainly related to endocrine disorders, when a man develops mammary hyperplasia in one breast, symptoms such as breast pain, breast nodules or lumps, and breast overflow may appear. 3. Mastitis: When men suffer from mastitis, it can also lead to hard lumps on one side of the breast and not on the other. The disease is associated with autoimmune factors, bacterial infections, breast trauma, etc. It is also easily accompanied by increased breast temperature, breast redness, swelling of local lymph nodes and other symptoms. 4. Breast cancer: the cause of the disease is not clear, but may be related to heredity, physical stimulation, obesity and so on. Early symptoms of most patients are not obvious, with the progress of the disease can appear breast lumps, breast skin abnormalities, nipple discharge and other symptoms, which can be manifested as hard lumps on one side of the breast and not on the other side. In addition, breast trauma, breast fibroma, breast cysts, extramammary diseases and other reasons may also appear similar clinical manifestations, it is recommended that boys with hard lumps on one side of the breast and not on the other side of the medical examination, in order to clarify whether it is caused by breast diseases. Pay attention to breast care in daily life and try to wear loose clothing.