This meta-analysis has the aim of measuring the effect of treatment on sex offenders. After a systematic review of the recent literature, seventeen studies were selected, containing a total sample of 6,681 sex offenders. The rates of sexual recidivism (13.12% vs. 17.94%), violent ─including sexual─ (25.5% vs. 29.1%) and general ─any type of recidivism─ (46.53% vs. 52.41%) of treated offenders were less than those of the control groups. The effectiveness of the treatment was clear in reducing the rates of sexual (OR = .69; p < .05) and general (OR = .66; p < .05) recidivism of the subjects treated, but not the rates of violent recidivism. This results demonstrated the abilty of psychological treatments for reducing the risk of sexual and general recidivism of sex offenders. However, the interpretation of such results requires caution, given that an independent analysis of the studies of a good methodological quality did not show significant effects of treatment. The need for new and better comparison studies to assess treatment effect is clear, especially in Europe.