To study whether benign prostatic growth in aging men correlates with an increase in proliferation, proliferation rates were determined immunohistochemically using the antibody Ki-67 in 20 benign hyperplastic prostates (BPH) and in four normal prostates (NPR). There was no significant correlation between age and proliferation rate in epithelium or stroma in BPH. In addition, no significant correlation between prostate weight and proliferation rate could be demonstrated in either compartment. In NPR the proliferation rate in epithelium and stroma was 9 times and 37 times lower, respectively, than in BPH. Obviously the induction of BPH from NPR may be associated with a distinct increase in proliferation. The further increase in BPH volume, however, is not correlated with a further increase in proliferation rate.