The goal of the study was to provide a quantitative analysis of the relative efficacy of all five currently available serotonin reuptake inhibitors (SRIs) and behavior therapy [exposure and response prevention (ERP)] for obsessive compulsive disorder. The relationship between effect size and methodological characteristics was also empirically examined. A search was conducted of several computerized databases covering the dates from 1973 to 1997. Seventy-seven studies were identified, yielding 106 treatment comparisons involving 4641 patients. Effect sizes were analyzed between individual interventions and between intervention class [SRI, ERP or the combined treatment of an SRI with ERP(ERP/SRI)]. Data were analyzed both before and after controlling for methodological variables. The effect size for clomipramine (CMI) was significantly greater than the other SRIs, with the exception of fluoxetine (FLX). CMI was not significantly greater than ERP or ERP/SRI. As a class, ERP was significantly greater than SRIs as a whole. Effect sizes were larger for studies without a control group or random assignment, for self-reported outcome measures, and varied significantly by method of effect size calculation. Year of publication was significantly related to effect size. When controlling for these methodological variables, CMI was not significantly greater than FLX or fluvoxamine (FLV), and ERP was no longer significantly greater than the SRIs as a whole. No significant difference was found between CMI and the other SRIs as a group in head to head trials. No differences in drop-out rates were found. CMI stands out from the other SRIs. This difference is probably not clinically significant enough to warrant first choice treatment, given CMI's greater lethality in overdose. The choice between an SRI or ERP is dominated primarily by the infrequent availability of ERP and to a lesser degree by personal preference. Methodological differences significantly impact effect size.