Of all markers associated with cellular proliferation in breast carcinoma, Ki-67 has more often been correlated with prognosis in patients with these tumors than others. To investigate the relevance of Ki-67 determination at each phase of the cell cycle in the biological assessment of mammary carcinoma we applied bivariate Ki-67/DNA content analysis on samples from 154 resected primary lesions. Three Ki-67-derived indices including an overall and G1 and S+G2M indices were generated. These values were correlated with similar indices derived from flow cytometric DNA/RNA analysis and traditional clinicopathologic factors. The results show that overall Ki-67 indices do not correlate with flow cytometric values and clinicopathologic factors. Flow-derived Ki-67 and DNA S+G2M indices were positively correlated (p<0.0001, r=0.58). High indices for the S+G2M phase derived by both Ki-67 and DNA analysis were significantly correlated with DNA aneuploidy, high tumor grade, and negative hormonal status. We conclude that the proliferative fraction (S+G2M) by either Ki-67 or DNA analyses offers more practical and clinically relevant information in assessing the proliferative activity in mammary carcinoma.