BACKGROUND: There is evidence that offenders consume diets lacking in essential nutrients and this could adversely affect their behaviour. AIMS: To test empirically if physiologically adequate intakes of vitamins, minerals and essential fatty acids cause a reduction in antisocial behaviour. METHOD: Experimental, double-blind, placebo-controlled, randomised trial of nutritional supplements on 231 young adult prisoners, comparing disciplinary offences before and during supplementation. RESULTS: Compared with placebos, those receiving the active capsules committed an average of 26.3% (95% CI 8.3-44.33%) fewer offences (P=0.03, two-tailed). Compared to baseline, the effect on those taking active supplements for a minimum of 2 weeks (n=172) was an average 35.1% (95% CI 16.3-53.9%) reduction of offences (P<0.001, two-tailed), whereas placebos remained within standard error. CONCLUSIONS: Antisocial behaviour in prisons, including violence, are reduced by vitamins, minerals and essential fatty acids with similar implications for those eating poor diets in the community.