To assess the effectiveness of an alcohol brief intervention administered to violent offenders in a judicial (Magistrates' court) setting, a randomised controlled trial was conducted. Participants (n=269) were men aged 16-35 years, who resided within a 30-mile radius of Cardiff, UK, and who had been sentenced for a violent offence committed whilst intoxicated with alcohol. Recruitment occurred on-site. Participants were randomly allocated to receive an alcohol brief intervention (n= 135) or to a control group (n=134). Objective measures comprised re-offending and injury rates. Self-reported outcome measures comprised Alcohol Use Disorders Identification Test (AUDIT); number of drinking days in the past 3 months; total weekly standard units of alcohol; and stage of change with respect to alcohol consumption (as measured by the Readiness to Change Questionnaire). Participants were followed up 3 months (n=234; 87%) and 12 months (n=202; 75.1%) after sentence. No significant between-group differences were observed in any of the alcohol measures or in re-offending. Injury was significantly less likely in offenders who had received the intervention (27.4%) than those who had not [39.6%; 95% confidence interval (CI)=−0.23, −0.009]. At 3-month follow-up, significantly more participants in the intervention group (31%; n=37) than control group (16%; n=18) demonstrated an increase in their readiness to change drinking behaviour (χ 2 =8.56; df=2; P=0.014), but this did not persist at 12-month follow-up.