The DNA ploidy pattern from fresh frozen specimens and survival rate was investigated in 91 primary breast cancers. Diploid patterns were found in 32 (35.2%) and aneuploid patterns in 59 (64.8%). The 5-year overall survival rate was significantly lower in aneuploid cases (76.3%) than diploid cases (93.8%) (p = 0.042), while there was no significant difference in disease-free survival between the two groups. there were negative nodes, no significant differences in 5-year overall or disease-free survival between patients with diploidy and aneuploidy. In contrast, when there were positive nodes, the 5-year overall and disease-free survival rates in patients with aneuploidy were 60.6% and 48.5%, which were significantly lower (p = 0.048 and p = 0.030) than the corresponding percentages of 92.3% and 84.6%, in those with diploidy. When the ploidy pattern was compared with other factors, a very close correlation was found between the ploidy pattern and histological grading (p < 0.0001). The ploidy pattern determined by flow cytometric DNA analysis may reflect the grade of malignancy of the breast cancer.