Epidemiological reports suggest that Asians consuming a diet high in soy have a low incidence of clinically manifested prostate cancer. We have tested the hypothesis that life-time exposure to genistein, the primary isoflavone component of soy, is responsible for this protective effect. Lobund-Wistar rats were exposed to 0, 25 and 250 mg genistein/kg AIN-76A diet, starting at conception and continued until necropsy at 11 months. Male offspring were injected s.c. with Flutamide on days 50-66 and with testosterone on days 67-69, injected with N-methylnitrosourea (NMU) into the dorsal prostate on day 70, and given testosterone implants, starting at day 77. Genistein in the diet inhibited the development of NMU-induced prostate invasive adenocarcinomas, in a dose-dependent manner. Genistein did not alter body, prostate and testes weights. Male rats fed 0, 25 and 250 mg genistein/kg diet had serum genistein concentrations of 9, 60 and 861 pmol/ml, and prostate genistein concentrations of 85, 230 and 775 pmol/g tissue. We conclude that lifetime dietary genistein protected against chemically induced prostate cancer development in rats.