Epidemiological studies have yielded conflicting results on the associations of diet with prostate cancer. We review evidence that Brassica vegetables are associated with reduced prostate cancer risk. Brassica vegetables, which include broccoli, cabbage, mustard and collard greens, and bok choy, contain glucosinolates, the metabolic breakdown products of which are potent modulators of xenobiotic-metabolizing enzymes that protect DNA from damage. Twelve published studies give some information about Brassica vegetables and prostate cancer risk; six of these studies can be clearly interpreted. Of these, three reported statistically significant reduced risks (P < 0.05) and one reported a borderline significant reduced risk (P = 0.06) with high Brassica vegetable consumption. The epidemiological literature provides modest support for the hypothesis that high intakes of Brassica vegetables reduce prostate cancer risk.