ObjectivesTo assess the impact of a short psychoeducation intervention for antisocial personality disorder on offending after randomization to treatment.DesignMulticentre, superiority, non-blinded randomized controlled trial. Random assignment was conducted in blocks of varying sizes at a central randomization centre. Participants were followed using national register data until 365 days after randomization, migration, or death, whichever occurred first.SettingThirteen outpatient uptake areas in Denmark.ParticipantsPatients with antisocial personality disorder in treatment for substance use disorders were randomized to treatment as usual (TAU, n = 80) or Impulsive Lifestyle Counselling (ILC, n = 96). A total of 165 patients could be linked to criminal records (TAU, n = 74; ILC, n = 91).InterventionILC is a brief psychoeducational program targeting antisocial behavior. The trial was conducted between January 2012 and June 2014.OutcomesNumber of criminal offences leading to convictions based on national registers.ResultsThe mean number of offences was 2.76 in the TAU group (95% Poisson confidence interval [CI] = 2.39, 3.16) and 1.87 in the ILC group (95% CI = 0.97, 1.43). Negative binomial regression was used to assess total number of convictions, as well as convictions for violent, property, driving under the influence, and drug-related crimes. In both adjusted and unadjusted analyses, random assignment to ILC was associated with a lower number of total offences (incidence rate risk ratio [IRR] = 0.43, p = .013; adjusted IRR = 0.45, p < .001) and convictions related to violence (IRR = 0.19, p = .001 adjusted IRR = 0.19, p = .007) and property offences (unadjusted IRR = 0.30, p = 0.003, adjusted IRR = 0.42, p = 0.010). Differences between conditions were not significant for driving under the influence (unadjusted IRR = 0.49, p = .370; adjusted IRR = 0.53, p = .417) or drug offences (unadjusted IRR = 1.06, p = .907; adjusted IRR = 0.55, p = .223).ConclusionsThe ILC program shows promise in reducing offending behavior in people with comorbid substance use and antisocial personality disorder.Trial registrationISRCTN registry, ISRCTN67266318, 15/10/2012.