Breast pain on one side before menstruation is most likely lobular hyperplasia, but there is also a possibility of breast cancer. Lobular hyperplasia is caused by an imbalance in the ratio of estrogen to progesterone, resulting in an overgrowth of the breast parenchyma. In some cases, there are abnormalities in the quality and quantity of female hormone receptors due to the uneven composition of the parenchyma, so that the various parts of the disease are unevenly proliferated. Clinical manifestations of lobular hyperplasia of the breast lies in one or both breasts swelling and pain as the main symptom, some patients have cyclic pain, obvious pain before menstruation, pain after menstruation obviously reduce, serious cases will be painful throughout the month. Prevalent in middle-aged women. Before menstruation, pain in one breast should also be considered as mastitis, breast trauma and extra-mammary diseases, etc. Therefore, it is recommended to go to the hospital to improve the examination. For example, ultrasound and molybdenum scan can help to identify them.