Escitalopram, a selective serotonin reuptake inhibitor (SSRI), was compared to placebo in a study of patients with major depressive disorder (DSM-IV) who had baseline Montgomery-Asberg Depression Rating Scale (MADRS) total scores >or=22 and <or=40. After a 1-week, single-blind placebo period, patients were randomized to receive escitalopram 10 mg/day (n=191) or placebo (n=189) in an 8-week, double-blind period. The primary efficacy analysis of adjusted mean change in MADRS total score from baseline showed a statistically significantly larger effect for escitalopram than for placebo with a treatment difference at week 8 (last observation carried forward, LOCF) of 2.7 points (SE 0.85; P=0.002). In further by-week efficacy analyses, the effect of escitalopram was consistently larger than that of placebo (P<0.05) beginning at week 1 (Clinical Global Impression-Improvement score), week 2 (MADRS score) or week 3 (Clinical Global Impression-Severity score). Escitalopram was very well tolerated with a low overall withdrawal rate similar to that for placebo. Nausea was the only adverse event reported significantly more in escitalopram-treated patients than in placebo-treated patients, although it was infrequent and transient. Escitalopram 10 mg/day had a statistically significantly better antidepressant effect than placebo as early as week 1, and was safe and very well tolerated.