Yellow water coming out of the left breast is usually referred to as nipple discharge, which may be related to pregnancy, cystic hyperplasia or ductal dilatation of the mammary gland. 1. Pregnancy: Nipple discharge can occur during pregnancy, resulting in yellow water coming out of the left breast, which is a normal physiological phenomenon and does not require excessive concern. 2. Breast cystic hyperplasia: When fluid accumulation occurs in the patient’s breast, it may cause breast cystic hyperplasia. The disease usually has no obvious symptoms, but when the left breast is squeezed, the fluid in the breast can be discharged from the body and cause the above symptoms. 3. Dilatation of mammary ducts: Dilatation of mammary ducts can be triggered by factors such as breastfeeding disorders and bacterial infections. This disease will lead to simple nipple overflow, resulting in the symptom of yellow water coming out of the left breast, which may be accompanied by itching sensation of the nipple. Yellow water coming out of the left breast may also be caused by breast cancer, hyperpituitarism, etc. Patients should go to the hospital for timely treatment.