It was demonstrated that soy isoflavone intake was associated with a reduction in prostate cancer diagnosis in men with a clinically confirmed risk of prostate cancer (risk ratio = 0.49, 95% CI = 0.26–0.95), but no significant results were demonstrated regarding the relationship between the intake of soy isoflavones and the serum PSA level in men with confirmed prostate disease. This evidence concerns the gene KLK3 and prostate cancer.