Swelling on the left side of ductal carcinoma in situ may be caused by enlarged lump of carcinoma in situ, acute mastitis, breast hyperplasia and so on. 1. Lump enlargement of carcinoma in situ: due to the location of the disease in the internal duct of breast, the blockage of the duct is not smooth, resulting in the formation of lumps in the breast, and with the increase of the lumps, further compression of the surrounding tissues, which can lead to swelling of the breast. 2. Acute mastitis, commonly found in breastfeeding women, is mainly due to bacterial infection of the breast tissue through the cracked nipple, initially there is swelling and pain in one side of the breast or unilateral breast localized skin temperature increases. 3. Breast hyperplasia, caused by endocrine hormone disorders, clinical manifestations of breast swelling and pain with breast hyperplasia nodule formation, and the nature of the pain and the size of the nodule with the menstrual cycle changes. The nature of pain and the size of the nodule are related to the change of menstrual cycle. Swelling on the left side of ductal carcinoma in situ should also be considered as other reasons such as trauma, which should be taken seriously, and the patient should go to the hospital in time to take targeted treatment measures after clarifying the cause of the disease.