There is evidence that DNA quantization and histopathological classification of breast cancer may be useful for its therapeutic management. DNA flow cytometry, clinical and anatomopathological features of 60 paraffin embedded primary breast cancer tissue samples were studied. The aneuploidy percentage was 67%. There was a correlation between DNA index and degree of cellular pleomorphism, degree of differentiation and the fraction of cells in S phase. Likewise a correlation was found between the degree of cellular pleomorphism and the mitotic index. DNA cytophotometry was useful to solve cases of difficult diagnosis with flow cytometry (''near'' diploid and tetraploid tumors). No correlation was found between aneuploidy, percentage of cells in phase S, degree of cellular atypia and mitotic degree with clinical stage, degree of lymph node involvement, tumoral size or age. It is suggested that these variables may have an independent behavior.