Despite the growth in literature on the merits of treatment, opinion remains divided about the lack of relevant evaluative research on the effectiveness of psychological treatments for sexual offenders. This study reports a systematic review of randomised control trials reporting the effectiveness of sexual offender treatment programmes. Electronic and hand searches were carried out for randomised control trials published between 1998 and 2003. Searches revealed nine randomised control trials (RCTs) and these contained data on the treatment of over 500 men, 231 of whom have been followed up for 10 years. Analysis of the nine trials showed that cognitive behavioural therapy (CBT) in groups reduced re-offence at one year compared with standard care (n=155) but increased re-arrest at 10 years. The implications of these findings and the subsequent use of randomised control trials in clinical policy and research are discussed.