Nuclear DNA content in 35 cases of breast carcinomas was determined by flow cytometry using paraffin-embedded archival tissue, in order to investigate the correlation between DNA ploidy and pathologic parameters, such as histologic type and degree of differentiation. DNA aneuploidy was found in 18 cases(51.4%). DNA ploidy was correlated with histologic type, tumor grade, and degree of tumar necrosis(P<0.005), but not with patient age, tumor size, or axillary nodal status(P>0.1). There was a tendency for high grade diploid tumors to have a higher S-phase fraction than low grade tumors, although it was statistica11y insignificant. Four patients out of 18 with aneuploid tumors died of breast cancer 10 months to 2 years after operation. However, it was impossible to analyse clinical and pathologic parameters to predict biologic behavior because of the small number of cases and inadequate follow-up data.