Flow cytometric DNA analysis using paraffin-embedded tumor blocks was done retrospectively on 155 node-negative breast cancers. The median duration of follow-up in patients still alive at the time of analysis was 10 years. Tumor aneuploidy was correlated significantly with increased tumor size (P = 0.003) and higher tumor grade (P less than 0.001). No significant correlation between tumor ploidy and patient age was found. Patients with diploid tumors had a significantly improved relapse-free and overall survival compared with patients with aneuploid tumors (P = 0.0001). In a Cox multivariate model with parameters including ploidy, histologic grade, tumor size, and patient age, ploidy (P = 0.02) and tumor size (P = 0.05) emerged as significant independent predictors of overall survival. Only ploidy was independently significant in the analysis of relapse-free survival. In conclusion, the current study indicates that flow cytometric measurement of DNA ploidy is a powerful prognostic indicator in node-negative breast cancer patients.