Background: Aggression replacement training (ART) is a widely used cognitive behavioural intervention for reducing aggression-related recidivism among criminal offenders. Its effectiveness in reducing offending, however, remains uncertain. Aim: To examine the effect of ART on adult offenders' criminal recidivism rates. Method: We compared 1,124 convicted adult offenders who began ART in the Swedish Prison and Probation Services 2003-2009 with 3,372 offenders in the system at the same time who did not participate in ART. Linkage with nationwide, longitudinal registries allowed extensive propensity score-matched controlling for baseline differences (e.g. sociodemographics, criminal history, psychiatric morbidity, and substance misuse) between groups.Results: Intent-to-treat analyses suggested similar 1-year