A meta-analysis was conducted to integrate treatment results from outcome research on the efficacy of antidepressants, behavior therapy, cognitive therapy, and the combination of these methods in obsessive compulsive disorder. The effect sizes for self-rated obsessive compulsive symptoms were found to be significantly smaller than for assessor-ratings. Because of the lack of overlap in use of measurement instruments in the respective studies, no common measurement instrument could be selected. Therefore, analyses were performed on both self- and assessor-ratings. On self- and assessor-rated obsessive-compulsive symptoms, the serotonergic antidepressants clomipramine, Fluoxetine, and fluvoxamine, behavior therapy, and the combination of serotonergic antidepressants with behavior therapy were significantly more effective than placebo treatment. Although scarce, follow-up data from 3 months to 6 years indicated that the short-term treatment effects remain stable. On self-ratings, behavior therapy was significantly more effective than serotonergic antidepressants. The combination treatment tended to be more effective than serotonergic antidepressants. On assessor-ratings, however, no difference could be demonstrated between these three treatments. Future research on antidepressants and behavior therapy should adopt the same standards for self and assessor-ratings. Moreover, research should focus on long-term efficacy of treatments offered.