Imprint smears from sixty cases of breast cancer made after mastectomy were stained by the Feulgen method and the nuclear DNA content measured by a cytofluorometer equipped with an incident illumination system. After logarithmic transformation of the fluorescence intensity, the ploidy level and S-phase fraction (SPF) were calculated with a microcomputer and the correlation between the ploidy level or SPF and the clinicopathological prognostic factors studied. Patients with tumors of a larger diameter and more extensive lymph node involvement had higher levels of ploidy and SPF and the ploidy level in the metastatic lymph nodes was higher than that in the primary lesion. Moreover, a significant increase in SPF was observed in the metastatic lymph nodes and a high ploidy level found to be associated with tumors having a negative estrogen receptor. When the tumors were divided into a diploid group and an aneuploid group, the diploid group showed a significantly better prognosis than the aneuploid group, in 6-year survival. Similarly, the groups in which SPF was less than 20.0 per cent had significantly better prognoses than the group in which SPF was 20.1 per cent or more. These prognostic factors were evaluated with Cox's proportional hazard model and a significant correlation observed in lymph node status, ER status, ploidy level and S-phase fraction. It was thus concluded that ploidy level and SPF are important and independent prognostic factors for predicting the postoperative course of breast cancer patients.