Learning Objectives Explain the prognostic value of baseline CTCs. Describe the predictive value of CTCs at day 21 after chemotherapy. Explain the meaning of the change of CTCs from baseline to day 21 after chemotherapy. We investigated the prognostic significance of circulating tumor cells (CTCs) determined immediately before the second cycle of chemotherapy in patients with metastatic breast cancer (MBC). The CTC counts were taken at baseline, before the first cycle of chemotherapy (CTC‐0), and on day 21 before commencing the second cycle of chemotherapy (CTC‐21) in consecutive MBC patients. The study's primary objectives were to analyze relationships between CTC‐21 count and overall survival (OS). Based on the current literature, the CTC measurements were dichotomized as 0–4 versus ≥5 CTCs. Of 117 patients recruited, 99 were evaluable. Patients with 0–4 CTCs on day 21 had a significantly better OS than those with ≥5 CTCs (median OS: 38.5 months vs. 8.7 months). They also had a significantly better progression‐free survival (PFS; median: 9.4 months vs. 3.0 months) and clinical benefit rate (77% vs. 44%). The OS of patients whose baseline CTCs were ≥5 but dropped to <5 on day 21 was apparently similar to those who had <5 CTCs at baseline. In a Cox regression analysis, CTC‐21 was the only independent variable significantly predicting OS and PFS. Our data indicate that CTCs determined immediately before the second cycle of chemotherapy is an early and strong predictor of treatment outcome in MBC patients. This study investigated the prognostic significance of circulating tumor cells (CTCs) determined immediately before the second cycle of chemotherapy in patients with metastatic breast cancer (MBC). The data indicate that CTCs determined immediately before the second cycle of chemotherapy is an early and strong predictor of treatment outcome in MBC patients.