Cellular DNA content of 72 samples was analyzed by flow cytometry. The results showed that 20 benign tumors and 10 normal breast tissue specimens were found to be DNA diploidy. 26 (62%) of 42 cases of breast cancer were DNA aneuploidy and the remainders were DNA diploidy. The median s-phase cell percentage was significantly higher in aneuploid tumors (28%) than diploid tumors (13%) and benign tumors (4%). Estrogen receptors were also investigated in 42 breast cancer specimens. There were 31 ER positive (74%) cases and 11 negative (26%) cases. The aneuploid DNA content was usually associated with lack of estrogen receptors. DNA content correlated significantly with age and ER status. The results suggest that flow cytometric analysis of cellular DNA content of breast cancer and status of estrogen receptors can provide an objective quantitative and reproducible new parameter for predicting prognosis of breast cancer.