Flow cytometry analysis of cell cycle distribution was carried out on acridine-orange-stained cell suspensions freshly prepared from carcinoma of the breast of 90 women, and compared with control, benign breast specimens from 10 women. There was a significantly greater proportion of cells in S (9.1% +/- 5.8) and G2 + M (8.0 +/- 6.0) phases of the cycle in breast cancer specimens than in the S (1.5% +/- 0.6) and G2 + M (2.5 +/- 0.9) phases of the cycle in benign, control specimens of breast tissue. Great variation was noted among the breast cancers, with medullary carcinomas having the highest percentage of cells in S + G2 + M, and papillary, tubular, colloid and well-differentiated duct carcinomas having the lowest. Tumors from premenopausal women classified as estrogen receptor negative had significantly more cells in S + G2 + M than did tumors from postmenopausal women that were estrogen-receptor positive.