We investigated whether vitamin-mineral supplementation could reduce serious rule violations. In this randomized, controlled, double-blind trial, young adult male inmates were included. For 15 weeks, they received a daily dose with vitamin-mineral supplements of approximately 100% Recommended Dietary Allowance (RDA) ( n = 149), or a higher-dose formula ( n = 150), or placebo ( n = 150). Serious rule violations were the primary outcome. In the lower-dose group there were 39% fewer rule violations than in the placebo group (relative risk = .61; 95% confidence interval [ CI]:.41–.90, p = .01). In the higher-dose group the difference did not reach statistical significance. About 100% RDA, but not higher doses supplements, resulted in less serious rule violations than placebo.