Based on epidemiological surveys, the low incidence of clinical prostate cancer among aged men in Japan and China were attributed to high consumption of soybean-derived food in which phytoestrogens have numerous anticancer mechanisms. The prostate model in Lobund-Wistar (L-W) rats produce high levels of testosterone (T). They are inherently predisposed to develop induced and spontaneous metastasizing adenocarcinomas, which are (T)-dependent in early stages and T-independent in advanced stages. In the experiment reported here, 2 groups of L-W rats (age 2 months) were fed soy-containing diets: (a) commercial diet L-485 (TekLad) with soy meal; or (b) a soy-free diet (L-474) in which casein was replaced by soy protein isolate/isoflavones (SPII). At age 3 months, all rats were inoculated i.v. with MNU; and 14 months later, 17/58 (29.3%) of rats on diet L-485 developed cancer in avg 12 months, compared to 5/50 (10%) of rats on the SPII diet in avg 12.1 months (P = 0.001). In the latter rats, the serum levels of T, and weights of testes were significantly reduced; but in the former rats, serum levels of T remained elevated, suggesting that soy meal in L-4,85 blocked the estrogenic effects of phytoestrogens.