The DNA content of paraffin-embedded tumor tissue has been measured by flow cytometry in 169 patients with operable breast cancer Stage I-II. The medium follow-up period was 123 months. Aneuploid primary tumors were found in 49% of patients. Tumor ploidy significantly correlated with histological type of tumor (p < 0.05), whereas no clear correlation between DNA ploidy and tumor size, histological grade and lymph node involvement was found. After 10-year follow-up, recurrence-free survival (RFS) of patients with diploid tumors was slightly better than the survival of those with aneuploid tumors, but the difference was not statistically significant (p = 0.39). In a Cox multivariate analysis only the axillary lymph node involvement and tumor size proved to be independent prognostic factors for recurrence, whereas DNA ploidy lost its prognostic value already in the univariate analysis. Therefore, we can conclude that the information on DNA ploidy, obtained from archival material, does not contribute significantly to better discrimination between good-risk and poor-risk operable breast cancer patients.