Description: Purpose: Although unequivocal evidence has shown the prognostic relevance of circulating tumor cells (CTC) in the peripheral blood of patients with metastatic breast cancer, less evidence is available for the prognostic relevance of CTCs at the time of primary diagnosis. Experimental Design: We conducted a pooled analysis of individual data from 3,173 patients with nonmetastatic (stage I–III) breast cancer from five breast cancer institutions. The prevalence and numbers of CTCs were assessed at the time of primary diagnosis with the FDA-cleared CellSearch System (Janssen Diagnostics, LLC). Patient outcomes were analyzed using meta-analytic procedures, univariate log-rank tests, and multivariate Cox proportional hazard regression analyses. The median follow-up duration was 62.8 months. Results: One or more CTCs were detected in 20.2% of the patients. CTC-positive patients had larger tumors, increased lymph node involvement, and a higher histologic tumor grade than did CTC-negative patients (all P 〈 0.002). Multivariate Cox regressions, which included tumor size, nodal status, histologic tumor grade, and hormone receptor and HER2 status, confirmed that the presence of CTCs was an independent prognostic factor for disease-free survival [HR, 1.82; 95% confidence interval (CI), 1.47–2.26], distant disease-free survival (HR, 1.89; 95% CI, 1.49–2.40), breast cancer–specific survival (HR, 2.04; 95% CI, 1.52–2.75), and overall survival (HR, 1.97; 95% CI, 1.51–2.59). Conclusions: In patients with primary breast cancer, the presence of CTCs was an independent predictor of poor disease-free, overall, breast cancer–specific, and distant disease-free survival. Clin Cancer Res; 22(10); 2583–93. ©2016 AACR .