OBJECTIVES: To determine the effects of diets rich in soy and linseed compared with a control diet on biochemical markers of prostate cancer in men diagnosed with prostate cancer. METHODS: Twenty-nine men diagnosed with prostate cancer and scheduled to undergo a radical prostatectomy were randomized to one of three groups: soy (high phytoestrogen), soy and linseed (high phytoestrogen), or wheat (low phytoestrogen). A bread was specially manufactured to incorporate 50 g of heat-treated (HT) soy grits or 50 g of HT soy grits and 20 g of linseed as part of the study participant's daily diet. Baseline and preoperative levels of prostate-specific antigen (PSA), free PSA, testosterone, sex hormone-binding globulin, free androgen index, and dihydrotestosterone were measured. RESULTS: Statistically significant differences were detected between the HT soy grits group and the control wheat group for the percentage of change in total PSA (-12.7% versus 40%, P = 0.02) and the percentage of change in free/total PSA ratio (27.4% versus -15.6%, P = 0.01); and between the HT soy grits group and the HT soy grits and linseed group for the percentage of change in free androgen index (16.4% versus -15.5%, P = 0.04) and the percentage of change in free/total PSA ratio (27.4% versus -10%, P = 0.007). CONCLUSIONS: The data from this study indicate that a daily diet containing four slices of a bread rich in HT soy grits favorably influences the PSA level and the free/total PSA ratio in patients with prostate cancer. This work provides some evidence to support epidemiologic studies claiming that male populations who consume high phytoestrogen diets have a reduced risk of prostate cancer development and progression.