Left breast pain can be physiologic due to changes in hormone levels. It may also indicate the presence of glandular hyperplasia, mastitis, tumor, soft tissue injury and other diseases. 1. Glandular hyperplasia: Glandular hyperplasia is the most common factor leading to breast pain, which can be manifested as tingling, swelling and pain in the breast, and can be touched by a lump. 2. Mastitis: Mastitis is caused by bacterial infection or milk stagnation, which is characterized by pain on the affected side of the breast, accompanied by redness and swelling. 3. Tumor: Breast cancer can cause breast pain when pressing on the breast. 4. Soft tissue injury: localized breast pain can be caused by trauma or deep muscle strain. Left breast pain should also be considered as an extra-mammary disease. When persistent pain in the breast occurs, it should be diagnosed as soon as possible.