Chemopreventive constituents in food plants, such as brassica-derived isothiocyanates (ITC), have been shown to be quite effective in the prevention of genotoxic DNA damage in cell culture models and carcinogenesis in laboratory animals. We have conducted a controlled intervention study with 14 participants (10 female, four male) using DNA damage and micronucleus formation as intermediate endpoints to assess the chemopreventive nature of mustard. For this trial, human volunteers were fed 20 g (25 mg total ITC) of mustard preparation, daily, for 4 days. Heparinized blood was collected by venipuncture and processed for the comet assay or the micronucleus test. A 3-day intervention with mustard led to a significant reduction in DNA damage and micronucleus formation induced by hydrogen peroxide or benzo(a)pyrene diolepoxide. Clinical liver parameters were unchanged by the intervention; however, cholesterol levels were significantly reduced. The results of this study indicate that consumption of low amounts of ITC-containing mustard quickly and effectively modulates cytoprotective factors in peripheral blood mononuclear cells and/or blood. The fact that these observations were confirmed by two cytogenetic biomarkers for cancer risk implies that even short-term intake of ITC-containing vegetables might indeed be associated with reduced cancer risk.