The feasibility of using the monoclonal antibody Ki-67 as a proliferation marker in human prostatic carcinoma was studied on aspiration and core biopsy specimens obtained from 50 patients suspected of having prostate cancer. In 32 prostatic adenocarcinomas the Ki-67 index varied from 0.3 to 13.3% (mean 4.3) in cytological smears and from 0.8 to 17.8% (mean 5.1) in frozen sections from histological core biopsies. No significant correlation between the percentage of cells positive for Ki-67 and the histological tumor differentiation could be established. In 18 patients with benign prostatic hyperplasia the Ki-67 index varied from 0 to 3.0% (mean 1.2) and from 0 to 3.8% (mean 1.4) in cytological and histological material, respectively. The differences in the observed Ki-67 index between benign and malignant prostatic tissues are of statistical (p less than 0.001) and of clinical significance. Nine patients who underwent endocrine treatment or radiotherapy entered a followup protocol in which the Ki-67 staining procedure was applied to periodically obtained cytological aspiration biopsies. During month 1 after the start of therapy a statistically significant (p less than 0.05) decrease in the Ki-67 index to 58% of the initial values was found, while at 2 and 3 months the proliferative fraction showed a further decrease to 27 and 7%, respectively. As a marker, the monoclonal antibody Ki-67 was shown to provide a reliable method to estimate the proliferative cell fraction of human prostate cancer.