Differences between the side effect profiles of clomipramine (CMI) and the selective serotonin reuptake inhibitors may be important factors in both treatment outcome and patient selection in obsessive-compulsive disorder (OCD). Safety and efficacy data from an industry-sponsored, multicenter clinical trial of CMI were analyzed previously using tabular and multiple regression methods. Good response, defined as at least a 35% drop in final scores on the Yale-Brown Obsessive Compulsive Scale (Y-BOCS), was associated with a later age of OCD onset and certain early side effects that may reflect a sensitivity of responders to CMI's serotonergic actions. The authors conducted a similar analysis of data from an industry-sponsored clinical trial of fluoxetine in OCD. Fluoxetine response did not seem to be associated with age of OCD onset. Good response to both drugs was associated with initial nervousness and sexual complaints. The common side effects of fluoxetine (headache, nausea, and gastrointestinal complaints) did not seem to be associated with treatment response. Slight differences in the protocols of the two clinical trials yielded patient populations that were different in factors found to be associated with treatment outcome: subjects in the fluoxetine study had lower scores on the Y-BOCS, higher scores on the Hamilton Rating Scale for Depression, and an earlier age of OCD onset.