The purpose of this study was to evaluate the comparative efficacy and tolerability of sertraline and fluoxetine in the treatment of obsessive-compulsive disorder (OCD). Outpatients meeting DSM-IV criteria for OCD, with a Yale-Brown Obsessive-Compulsive (Y-BOCS) total score >or= 17, an NIMH Global Obsessive-Compulsive (NIMH-OC) scale score >or= 7, and a CGI-Severity score >or= 4 were randomized to 24 weeks of double-blind treatment with sertraline (N = 77) or fluoxetine (N = 73). Primary efficacy measures consisted of the Y-BOCS, the NIMH-OC scale, and the CGI-Severity (CGI-S) and Improvement (CGI-I) scales. Equivalent and significant (p < 0.001) improvement was found at week 24 in Y-BOCS and NIMH-OC scale scores for sertraline and fluoxetine. After 12 weeks, 49.2% of patients on sertraline were rated on the CGI-S scale as being mildly ill or not ill compared to 24.6% on fluoxetine (p < 0.01). A Cox analysis found patients on sertraline to have a statistically nonsignificant 42% greater likelihood of achieving a response by week 12 (CGI-I, much or very much improved; 95% CI, 0.85, 2.38; p = 0.18). Sertraline treatment also resulted in a higher proportion of remissions than fluoxetine (defined as a CGI-I <or= 2 and a Y-BOCS score <or= 11), both at week 12 (20% vs. 8%; chi2, 3.95; df 1; p = 0.047) and week 24 (36% vs. 22%; chi2, 3.18; df, 1; p = 0.075). Both medications were well-tolerated and demonstrated significant efficacy in the treatment of outpatients with moderate to severe OCD with the subjects treated with sertraline showing a greater likelihood of remission as well as an earlier improvement on some but not all efficacy measures