In situ determination of proliferative activity was performed on 203 breast cancers by use of Ki-67 monoclonal antibody and immunohistochemical methods. Tumor proliferation rate was analyzed and correlated to tumor size and nodal status. The relationship between Ki-67 proliferative activity and nuclear estrogen receptor content was also investigated on adjacent tissue sections. Ki-67 values ranged from 1 to 75%, with a median value of 10%. Premenopausal patients had greater Ki-67 values (median value, 14.1%) than postmenopausal ones (median value, 9.8%). The authors observed no correlation with lymph nodal involvement, whereas a statistically significant relationship with tumor size was found (P less than 0.01). An inverse correlation (Spearman's coefficient = -0.56; P less than 0.001) was seen between Ki-67 values and nuclear estrogen receptor content. These results, similar to those reported for other kinetic measurements, suggest that in situ detection of Ki-67 proliferation rate is a useful method for obtaining cell cycle information. Follow-up studies will be needed to assess an eventual prognostic relevance.