CA15-3 is the most important and specific marker for breast cancer. 30%-50% of breast cancer patients have significantly elevated CA15-3 and its level is closely related to the treatment effect, which is the best indicator for diagnosis and monitoring of post-operative recurrence and observing the efficacy of treatment in breast cancer patients. When CA15-3 is greater than 100U/ml, metastatic lesions can be considered. CA153 is valuable in the diagnosis and postoperative follow-up of breast cancer. The sensitivity is low in the early stage of breast cancer, about 60%. The positive rate of metastatic breast cancer can reach 80%. The waxing and waning of patient’s serum CA15-3 levels parallels the changes in breast cancer disease and is an important signal of recurrence and metastasis, and this signal is given earlier than the onset of clinical symptoms and the detection of recurrence and metastasis with, for example, ultrasound, X-ray or CT. It has been reported that when CA15-3 levels exceed 30u/ml, 40u/ml and 50u/ml, the sensitivity of determining the presence of postoperative local regional recurrence or distant metastasis in breast cancer patients is over 90%, the specificity is 95%, 99% and 100%, and the correct judgment rate is 56%, 83% and 100%, respectively.