Abstract: Objective To study the usefulness of changes in serum levels of the tumor markers CEA，CA₁₂₅，and CA₁₅₃ for assessing the efficacy of chemotherapy in patients with metastatic breast cancer without measurable disease. Methods Electronic medical records were retrospectively reviewed for 81 patients with metastatic breast cancer but without measurable disease treated between January 2008 and June 2013.Serum levels of CEA，CA₁₂₅， and CA₁₅₃ were measured with electrochemiluminescence before and after chemotherapy，and the levels were correlated with survival. Results Patients were grouped according to whether they presented normal levels of all three serum markers before chemotherapy （negative group，n=44） or they presented elevated levels of at least one marker before chemotherapy（positive group，n=37）.Median progression-free survival （PFS） was significantly longer in the negative group （19 months） than in the positive group（9.8 months，P=0.007）.The positive and negative groups had similar overall survival （51.9 vs 37.7 months，P=0.06）.Median PFS in patients in whom at least one tumor marker declined by one degree during chemotherapy（10.2 months）was longer than that in patients in whom at least one marker declined by two degrees（5.2 months，P=0.008）.Conclusions  Elevated levels of serum CEA，CA₁₂₅ or CA₁₅₃ correlate with shorter PFS in metastatic breast cancer patients without measurable disease.A decline of two or more degrees in the levels of any one of these tumor markers is a significant predictor of PFS benefit.

Key words: Breast neoplasm, Tumor marker, Unmeasurable disease, CEA, CA₁₂₅, CA₁₅₃