Purpose. The effectiveness of prison‐based cognitive‐behavioural treatment programmes was evaluated using reconviction as the outcome measure.Method. Reconviction rates were compared between two groups of adult male offenders who were serving a custodial sentence of 2 years or more in Her Majesty's Prison Service, England and Wales. The treatment group (N=667) consisted of offenders who had voluntarily participated in one of two treatment programmes that targeted 'cognitive deficits' related to offending behaviour. The comparison group (N =1,801) was made up of offenders who had not participated in the treatment programme but were “matched” to the treatment group on a number of empirically relevant variables.Results. Treatment produced a robust reduction in the probability of reconviction (p<.001) when other relevant variables were controlled for. For treated offenders, the percentage point reduction in reconviction was 14% in medium‐lowrisk offenders and 11% in medium‐high‐risk offenders.Conclusion. These outcome results demonstrate that the principles of effective practice in the field of offender rehabilitation, which were identified through meta‐analytical research predominately in North America, can be applied to a UK offender population to