The proliferative activity in 42 cases of breast cancer were assessed immunohistochemically using antibodies to proliferating cell nuclear antigen (PCNA) and Ki-67. These indices were correlated with the steroid receptor status, pathologic stage, and disease-free survival. There was a strong direct correlation between the two proliferation indices (r = .902, P < 0.001, Kendall's rank correlation). There was a weak correlation between the hormone receptor status and proliferation indices that was not significant when statistically tested. The cases were stratified into PCNA low proliferative index (PI) group (< 4.5% positive cells) and PCNA high PI group (> 4.5% positive cells). In the low PI group, five of 18 (28%) patients were node-positive in contrast to eight of 14 (58%) patients in the high PI group. After a follow-up period of 42-60 months, 14 of 19 (74%) patients in the low PI group were disease-free compared to 10 of 17 (53%) patients in the high PI group. The PCNA and Ki-67 proliferative indices appear to be of great prognostic value and may help identify a subset of breast cancer patients who should be given adjuvant therapy.