There is a special group of patients who are breastfeeding young mothers and have found a breast lump at the same time. In March this year, a young mother from the city to our department, she told us: I am 27 years old, only eight months after giving birth to a child, is still breastfeeding, two months ago found a lump on the breast, and is not very painful, at first thought it was a refusal of milk, did not pay attention to. Later, after hot compresses, with the milk collector to collect milk also did not see the lump smaller, to the nearby hospital to see. The doctor examined the lump and considered it to be milk retention or acute mastitis, and suggested stopping breastfeeding and anti-inflammatory treatment, which was not carried out due to the fact that I did not want to wean. Later, the doctor suggested ultrasound and molybdenum target examination. The ultrasound showed that the mass was cystic solid with moderate echogenicity, irregular dark area in the center, and hemicyclic blood flow signal in the interior; the molybdenum target examination showed an irregular mass in the breast with calcification, blurred edges, and different densities. Ultrasound and mammogram diagnosed breast hyperplasia or milk retention cyst, but breast cancer could not be ruled out. Our doctor suggested me to have the examination after weaning the child for three months, saying that ultrasound and mammogram during breastfeeding are sometimes incomplete because there may be calcified spots in breast milk retention or in breast hyperplasia, and there will be changes after stopping breastfeeding, so the benignity and malignancy of the lumps can not be clarified at this point in time. I am a little worried about the delay in treatment and would like to know if I should have surgery immediately and how to determine the benign or malignant nature of the lump. What kind of examination should I do now? After receiving the patient, we learned the patient’s medical history and the results of the examination and told the patient: in order to know the benign or malignant nature of the mass, we must immediately carry out fine-needle aspiration cytopathological examination, which can make a clear diagnosis, and the fine-needle aspiration examination is minimally invasive, painless, inexpensive, fast, and does not need to be weaned. After the patient agreed, we quickly performed a fine needle aspiration examination of her breast lump, and the results came out 20 minutes later. To our surprise and the patient’s family’s unacceptability, the results turned out to be breast cancer, which necessitated immediate surgery. Why would such a young and breastfeeding woman develop breast cancer? First, in recent years, the incidence age of breast cancer is getting younger and younger; second, the incidence rate of breast cancer is increasing year by year; third, the increase or fluctuation of estrogen and progesterone levels in pregnant and lactating women will cause the proliferation of breast tissues; fourth, it is easy for lactating women to consider breast milk refusal when lumps occur and neglect further examination; fifth, lactating mothers are mostly young women who usually do not think about breast cancer and it leads to underdiagnosis. This leads to under-diagnosis. Therefore, women who have found breast lumps, especially breastfeeding women who have found breast lumps should immediately go to the breast specialist in the hospital, and while doing mammography, especially cytology which can determine benign and malignant diagnosis should be carried out, which can clearly distinguish whether it is milk retention cysts, mastitis, lobular hyperplasia, fibroadenoma, intraductal papilloma, or breast cancer, so as to avoid misdiagnosis and delay in the treatment. Treatment.