Technical Abstract: Background: Soy is a major plant source of dietary protein to humans. Epidemiologic studies show that consumption of soy foods may be associated with a reduction in cancer risk in humans. Objective: The purpose of this study was to conduct a meta-analysis on the association between soy consumption and prostate cancer in men. Design: We systematically reviewed studies obtained through a thorough MEDLINE literature search and identified 15 publications (six cohort and nine case-control studies) on soy consumption and eight case-control studies on isoflavone intake in relation to prostate cancer risk. We conducted this analysis using a random-effects model in which studies with smaller standard error of estimate are given greater weight in the summary measure. Results: Our analysis of studies on soy intake yielded an overall risk estimate of 0.74 (95% confidence interval: 0.63, 0.89; P = 0.001). When separately analyzed, studies on non-fermented soy foods yielded an overall risk estimate of 0.70 (0.56, 0.88; P = 0.002) and those on fermented ones yielded 1.02 (0.73, 1.42; P = 0.922). The analysis of studies on isoflavones yielded an overall risk estimate of 0.88 (0.76, 1.02; P = 0.085). When analyzed on the basis of daily isoflavone intakes, we found an overall risk estimate of 0.63 (0.41, 0.96; P = 0.03) from studies at mg/day and 0.99 (0.85, 1.16; P = 0.909) from those at µg/day, respectively. Conclusions: Results of this analysis suggest that consumption of soy foods is associated with a reduction in prostate cancer risk in men. This protection may be associated with the type and quantity of soy foods consumed.