During non-pregnancy and non-lactation, fluid coming out of the nipple when squeezing is called nipple discharge. Nipple discharge is one of the common symptoms of breast disease, according to statistics, nipple discharge as the first symptom of the doctor accounted for 3-14% of breast disease, the incidence rate is second only to breast lumps and breast pain. 1, if the nipple discharge is single nipple discharge, most of them are related to the following kinds of breast diseases: (1) mammary duct dilatation: some patients suffering from this disease, the first symptom of nipple discharge in the early stage. The color of the overflow is mostly brown, and a few of them are bloody; the laboratory examination of the overflow shows that there are a lot of plasma cells and lymphocytes without tumor cells. The disease is most common in non-lactating or menopausal women over 40 years of age. There is a lump adhering to the skin in the areola area where the overflow occurs, often less than 3 centimeters in diameter, and the lymph nodes in the axilla on the same side may be enlarged, soft, and painful to the touch. In case of concurrent infection, the lump is localized with inflammation manifestations of redness, swelling, heat and pain. (2) Intra-papillary papilloma: this disease is common in 40-50 years old people, 75% of the tumor occurs in the parts adjacent to the nipple, the tumor is very small, with the tip and villi, and there are a lot of thin-walled blood vessels, so it is easy to bleed. Tumor cells can be found in the overflow. Sometimes, patients can carefully palpate the breast and find a cherry-sized lump under the areola, which is soft, smooth and movable. (3) Breast cystic hyperplasia: it is common in women of childbearing age. Some patients have yellow-green, brown, bloody or colorless plasma-like nipple discharge, and no tumor cells exist in the discharge. There are two characteristics of this disease: first, it is manifested as cyclic breast swelling and pain, occurring or aggravated in the pre-menstrual period, which is not minded by the patients in mild cases, but can affect the work and life in severe cases. Secondly, breast lumps are often multiple, can be seen on one side or both sides, can also be limited to a part of the breast or scattered throughout the breast. The lumps are nodular and of different sizes, tough and not hard, with no adhesion to the skin and unclear boundaries with the surrounding tissues, and the lumps may shrink after menstruation. (4) Breast cancer: some breast cancer patients have bright red or dark red nipple discharge, sometimes produce watery discharge, colorless and transparent, occasionally sticky, leaving no traces after discharge, and cancer cells can be found in the discharge. 45-49 years old and 60-64 years old are the two peaks of this disease. The onset of the disease is slow, and patients may find breast lumps unintentionally, which are mostly located in the inner upper limit or outer upper limit, painless and gradually enlarged. In the advanced stage, the lesion site appears orange peel-like skin changes and satellite nodules. Axillary lymph nodes are enlarged, hard, and fused with each other into a mass with the progress of the disease. 2, also need to pay attention to: (1) whether the overflow is true or false. True overflow means that the fluid flows out of the milk ducts. Pseudo-overflow is common in nipple inversion, due to the nipple epidermal shedding cells accumulated in the inversion, causing a small amount of liquid-like soybean dregs-like exudate, often with a foul odor. Once you pull out the inverted nipple and keep the area clean, the “overflow” will disappear. (2) Whether the overflow is bilateral or unilateral. Bilateral overflow is physiological, such as stopping breastfeeding within a year, most women will still have a small amount of milk secretion. In the middle and late stages of pregnancy, some pregnant women can squeeze out a small amount of light-colored colostrum from both breasts. A few women may also experience a short period of milk spillage after a strong orgasm due to highly engorged breast blood vessels, breast enlargement, and nipple erection. When women enter menopause, endocrine disorders may cause some women to secrete a small amount of milk. All of the above are physiological conditions, not pathological. However, bilateral nipple overflow can also be pathological, such as a disease called amenorrhea-milk overflow syndrome, which is caused by pituitary microadenomas, and is accompanied by amenorrhea, headache, narrowing of the visual field, and elevation of prolactin in the blood in addition to milk overflow. The diagnosis can be confirmed by CT scan of the brain. Another kind of double nipple overflow is seen in patients with oligomastia. (3) Whether the overflow is single or multiple holes. The nipple has 15 – 20 openings for milk ducts. When there is an overflow, it is necessary to observe from which one or how many openings the fluid overflows. A single pore overflow is usually an intraductal papilloma. Multi-hole overflow may be physiologic, drug induced, systemic benign disease, or breast hyperplasia. (4) Whether the overflow is self-expressed or squeezed. The former is mostly pathologic, and about 13% of breast cancer patients have a history of spontaneous overflow. Benign or physiologic overflow is more common after extrusion. (5) Trait of overflow. In different diseases of breast, the character of the overflow is not consistent. For example: ① Milk-like. It is mostly physiologic, such as after weaning or recent abortion, and is not a manifestation of cancer. ② Purulent overflow, mostly ductal dilatation, plasma cell mastitis. ③ Pale yellow overflow is the most common type of overflow, which is seen in almost all kinds of breast diseases, with breast hyperplasia being the most common. There are also some cases of intraductal papilloma or breast cancer. Therefore, it is something to be vigilant about. ④ Hemorrhagic overflow, which can be of different colors such as bright red, coffee, yellowish and brown. This kind of overflow is a danger signal and should be highly alert, of which 50–75% are intraductal papilloma and 15% are breast cancer. If bloody overflow occurs after menopause, 75% of them are breast cancer. ⑤ Clear watery overflow, colorless and transparent, occasionally sticky, leaving no trace after overflow. This kind of overflow may be a sign of breast cancer and should be further examined. In conclusion, nipple overflow is an important breast symptom, of which 10–15% may be breast cancer. It is important to go to the hospital with symptoms and have a smear cytology of the overflow done. Near-infrared breast scanning has a positive diagnostic yield of 80-95% for ductal lesions in the areolar region, and ultrasound and mammograms are also reasonably accurate. Selective ductal imaging of lesions is a commonly used test for nipple discharge and is of great value in the differential diagnosis of benign and malignant nipple discharge; it also provides the physician with an accurate localization of the extent of surgical excision.