Left breast nodule may be caused by breast hyperplasia, mastitis, breast cancer and other diseases. 1. Mammary hyperplasia: The cause of mammary hyperplasia is mainly related to endocrine dysfunction, such as estrogen/progesterone imbalance, prolactin elevation, etc. Patients may have left breast nodules, breast pain, lumps, etc. Some patients may combine with nipple discharge. Patients may have symptoms such as left breast nodules, breast pain, lumps, etc. Some patients may combine with nipple discharge. 2. Mastitis: Mastitis may be caused by low immunity, bacterial infection, drug stimulation, trauma, smoking and other factors. Patients may have symptoms such as breast lumps, breast hardness, breast pain and so on, which are manifested as nodules in the left side of the breast. 3. Breast cancer: the cause of breast cancer is not yet clear, it may be caused by over-nutrition, obesity, excessive alcohol consumption and other factors. Patients will have symptoms such as breast lumps and nipple overflow, and some of them may manifest as left breast nodules. The causes of left breast nodule also include breast fibroid, etc. Patients are advised to clarify the cause of the disease in time. At the same time, patients should actively cooperate with doctors to take relevant treatment measures.