BACKGROUND: Previous comparative studies of the selective serotonin reuptake inhibitors (SSRIs) have rarely included a placebo control group and have rarely demonstrated significant between-group differences. The study reported on here was a placebo-controlled comparison of the antidepressant effects of two SSRIs, citalopram and sertraline. METHODS: Three hundred twenty-three patients with DSM-IV-defined major depressive disorder were randomized to 24 weeks of double-blind treatment with citalopram (20-60 mg/day), sertraline (50-150 mg/day), or a placebo. The primary efficacy measure was the Hamilton Depression Rating Scale (HAMD) and the primary statistical analysis was an analysis of variance comparing the change from baseline to the last observation carried forward in each treatment group. RESULTS: Both citalopram and sertraline produced significantly greater improvement than placebo on the HAMD, the Montgomery-Asberg Depression Rating Scale, and the Clinical Global Impression Scale. Significant improvement was observed at earlier timepoints in the citalopram group than the sertraline group; however, sertraline treatment was associated with increased gastrointestinal side effects and a tendency toward early discontinuation, and analyses that excluded early dropouts revealed similar acute efficacy for the two active treatments. The Hamilton Anxiety Scale demonstrated a significant anxiolytic effect of citalopram, but not sertraline, relative to placebo. CONCLUSIONS: This study confirms the antidepressant efficacy of two SSRIs, citalopram and sertraline. It is hypothesized that the more consistent evidence of antidepressant activity that was observed early in treatment in the citalopram group was related to more pronounced antianxiety effects and better tolerability upon initiation of therapy.