Monoclonal antibody to proliferating cell nuclear antigen (PCNA) has been used to identify the growth fraction in ten cases of benign prostatic hyperplasia (BPH), in 20 prostatic microcarcinomas (PMC) and in 30 cases of infiltrating prostatic carcinoma (PC). Ten year follow-up was available on all cases by means of clinical, serological, radiological and echographic examinations. The percentage of PCNA-staining nuclei was independently counted by two observers. Statistical analysis showed significant differences between PCNA/cyclin score of BPH and PMC without recurrences with respect to those of PMC with progression and of PC. PCNA immunostaining may represent a reliable method for assessing cellular proliferative activity. It may be used as a more powerful diagnostic hallmark of PMC than patterns of non-malignant microglandular proliferation and is also a useful additional test for assigning histological grades to PMC and PC. Statistical analysis indicated that PCNA/cyclin index was an independent significant prognostic indicator of predicting malignant progression (P < or = 0.01) and survival rates (P < or = 0.05) of PC and PMC (> 5 mm diameter).