Along with various imaging modalities, serologic tumor markers such as CA 15-3 and CA 27.29 have been used for decades to monitor treatment response in patients with metastatic breast cancer (MBC). Despite the frequent use of these markers, they lack high sensitivity and specificity for breast cancer progression. The prognostic significance of these markers remains indeterminate because of the conflicting outcome of many clinical trials. The circulating tumor cell (CTC) test has recently been studied in clinical trials in patients with MBC. Some of the studies showed that high levels of CTCs are correlated with poor survival in MBC. An intergroup trial is underway to determine the implication of changing treatment based on the CTC level. This article will discuss the current data on these markers, with special emphasis on the CTC test. The potential clinical utility of these markers will also be discussed.