Patterns of cell proliferation in the prostate were compared between benign epithelium and dysplasia. Proliferating cell nuclear antigen (PCNA) immunostaining was used to quantitate proliferation, and basal cells were tallied separately from secretory cells with the aid of keratin immunostaining. Using a novel technique, absolute cell densities (cells/mm) were determined and used to calculate growth fractions. In benign epithelium, 83% of PCNA+ cells were basal cells, while only 7% of PCNA+ cells in dysplasia were basal cells and there was a clear separation between groups. This dramatic shift of the proliferative compartment to the secretory cells in dysplasia was accompanied only by a moderate increase in overall secretory cell density and moderate reduction in basal cell density, but these ranges overlapped those of benign epithelium. The median PCNA+ secretory cell "growth fraction" was 0.12% in benign epithelium and 1.06% in dysplasia. The findings presented give further support to the concept that dysplasia represents an evolutionary stage in the malignant transformation of prostatic epithelium. The patterns of change in PCNA immunostaining may reflect certain aspects of the biologic nature of malignant transformation.