In this study, we examined whether free DNA extracted from the plasma of breast cancer patients, characterized as tumor DNA, could predict the overall survival (OS) of breast cancer patients. In total, 147 patients and 35 healthy controls were studied. Plasma DNA was assessed in the same way as tumor DNA, following identification of similar alterations in polymorphic markers and TP53 gene mutations. Although OS was the main focus of this study, recurrence and disease-free survival (DFS) were also analyzed. In 61 of the 142 patients, with an average 58 months of follow-up, a similar molecular signature in tumor and plasma DNA was detected. OS was 71% (95% CI, 61%–81%), and distribution as regards tumor plasma DNA was 59% (95% CI, 45%–73%) for positive cases and 83% (95% CI, 73%–93%) for negative cases (P = 0.01). Univariate analysis revealed a predictive value for tumor plasma DNA (P = 0.018) hazard ratio (HR) 2.5 (95% CI, 1.2–5.3), while multivariate analysis did not (P = 0.3), HR 1.6 (95% CI, 0.6–4.4). DFS was 37% (95% CI, 19%–55%) for positive patients and 75% (95% CI, 57%–93%) for negative patients (P = 0.005). Among the 35 recurrences observed, 25 were positive for tumor plasma DNA and 10 were negative, (P < 0.001). These results indicate that tumor plasma DNA at diagnosis can serve as a prognostic marker of the OS of breast cancer patients. © 2006 Wiley-Liss, Inc.