Red blood in the breasts near the armpits may be physiological phenomenon or pathological phenomenon such as breast hyperplasia, mastitis, breast tumor and so on. 1. Physiological phenomenon: if the red blood appears before the onset of physiological period, and will be accompanied by breast pain and other phenomena, after the onset of physiological period to recover, may be a normal physiological phenomenon. 2. Pathological phenomenon: (1) breast hyperplasia: if the breast also contains a lump, the lump area has obvious pain, and there is nipple overflow and other phenomena, you need to consider that it is caused by breast hyperplasia. Patients with breast hyperplasia will have localized redness that will become more pronounced due to swelling of the breasts. (2) Mastitis: If accompanied by a series of phenomena such as redness and swelling of the breast, localized elevated skin temperature, localized pain, etc., it is considered to have mastitis, which may be related to bacterial infection or autoimmune diseases. (3) Breast cancer: If the patient has breast cancer, it will lead to a series of lesions in the breast, such as tumor enlargement, obvious pain in the breast, nipple blood overflow, orange peel-like skin changes and other symptoms, and if it spreads to the surrounding tissues, it may cause redness and other phenomena in the armpit area. (4) Other causes: such as trauma, subcutaneous petechiae and ecchymosis due to blood clotting disorders. Due to the many causes, it is recommended that patients actively consult the doctor for a clear diagnosis and then standardized treatment .