Studies in vitro suggest that osteopontin (OPN), an extracellular matrix protein secreted by macrophages infiltrating prostate tumors, and by tumor cells, may have a role in the transition from clinically insignificant tumors to metastatic prostate cancer (PC). Latent PC occurs at equal rates in Western and Asian men, but the incidence of advanced PC is many-fold higher in Western men. Our earlier studies in TRAnsgenic Mouse Prostate adenocarcinoma (TRAMP) mice showed that genistein, an isoflavone found in soybeans, lowered the incidence of advanced PC. This suggested that lower intake of dietary soy may be one possible cause for higher incidence of advanced PC in Western men. The objective of the present study was to test the hypothesis that genistein may exert its preventive effect by inhibiting OPN expression. From 5 to 28 wk of age, 80, 68, and 30 TRAMP mice were fed AIN-76A diet containing 0, 250, or 500 mg genistein/kg body weight, respectively. Organ weights were measured. The steady-state level of OPN mRNA was evaluated by RT-PCR in a longitudinal study in 74 TRAMP and 32 nontransgenic litter mates (NTM). Administration of 250 and 500 mg genistein/kg AIN-76A improved survival (P = 0.008 and P = 0.005, respectively) and reduced mean weight of prostates with poorly differentiated cancer (PD) (P < 0.001), as well as the mean weight of periaortic lymph nodes (LN), although the latter was not significant. OPN was upregulated 10-fold in PD compared with prostates with a lower pathological score from TRAMP or NTM of any age (P = 0.003). OPN mRNA levels in the dorsolateral prostate and metastasis to LN were significantly correlated (r = 0.643; P = 0.00006). Genistein had a dose-dependent, significant inhibitory effect on OPN transcript levels in prostates displaying advanced prostate cancer (PD; score 6; P = 0.05). Studies are consistent with the possibility that dietary genistein may delay the progression from benign to malignant tumors by inhibiting OPN expression. © 2005 American Society for Nutritional Sciences.