This article discusses the results of a meta-analysis of the effectiveness of psychological treatment, by itself or in combination with drugs, of the obsessive-compulsive disorder. Twenty-three European articles meeting the selection criteria were included, offering a total of 43 independent studies. Standardized mean difference was calculated between the pretest and postest means. The global mean effect size, d+ = 1.443, showed a clear efficacy for reducing obsessions and compulsions as well as symptoms of depression, anxiety, and social adjustment, although the latter was reduced to a lesser extent. The most effective treatments consisted of combining exposition and response prevention techniques or cognitive restructuring with antidepressants (d+ = 2.044 and d+ = 2.953, respectively), such as clomipramine or fluvoxamine. A predictive model of the efficacy is proposed as a function of the different treatments and the methodological quality of studies. Finally, the practical, clinical, and research implications of the results are discussed.