PURPOSE: The aim of study was to evaluate the usefulness of serum HER2 as a therapeutic response indicator in patients with HER2 positive metastatic breast cancer (MBC). MATERIALS AND METHODS: The levels of serum HER2 and CA15.3 were assayed in 148 serial serum samples from 50 HER2 positive MBC patients at both the baseline and follow-ups. The changes in the levels of serum HER2 and CA15.3 in relation to the tumor responses to the various chemotherapy regimens were monitored. RESULTS: The levels of serum HER2 and CA15.3 were elevated in 82% and 62% of tissue HER2 positive patients, respectively, prior to therapies, with the changes in both tumor markers showing statistical significance in relation to the tumor responses (p<0.01) in patients with elevated baseline serum markers. CONCLUSION: The level of serum HER2 could be a valuable response indicator, not only for trastuzumab containing therapy, but also for other common MBC chemotherapeutic agents. Also, as it is more frequently elevated, the serum level of HER2 may also be a more useful tumor marker than CA15.3 in HER2 positive MBC.