In this study the effects of a Western-style diet on epithelial cell proliferation in the prostate and bladder of C57BL/6J mice were investigated. The Western-style diet contained increased fat and low calcium and vitamin D, compared with AIN-76A control diet, at levels simulating human Western diets based on nutrient density. After feeding the Western-style and AIN-76A diets for 5 and 16 weeks, mice were infused with bromodeoxyuridine (BrdU) for 72 h using s.c. implanted osmotic pumps. Findings revealed that in bladder epithelium BrdU labeling indices were not significantly different (P > 0.05) between mice on the control and Western-style diets at both time periods. However, significant increases in BrdU labeling indices occurred in epithelial cells of the anterior (P = 0.024) and dorsal (P = 0.049) lobes, but not in the ventral lobe (P = 0.21), of the mouse prostate after feeding the Western-style diet for 16 weeks, compared with mice on the control diet. These findings demonstrate Western-style diet induction of epithelial cell hyperproliferation in anterior and dorsal lobes of the mouse prostate. The findings further suggest that these nutrients may have a role in human prostatic carcinogenesis, since the anterior and dorsal lobes of the mouse prostate are homologous with the human prostate in embryological origin and histological structure and carcinomas induced in rodent models have similar characteristics to those found in human prostatic cancer.