OBJECTIVE: A randomized, double-blind, 24-week-fixed-dose study comparing the efficacy and safety of escitalopram to that of citalopram was safety was conducted in primary care patients with moderate to severe major depressive disorder (MDD). RESEARCH DESIGN AND METHODS: This was a randomized, double-blind, 24-week fixeddose study. Patients were randomly assigned to treatment with escitalopram 10 mg/day (n = 175) or citalopram 20 mg/day (n = 182). Clinical response was evaluated using the Montgomery-Asberg Depression Rating Scale (MADRS) and Clinical Global Impression-Severity (CGI-S) scale. The prospectively defined primary parameter of antidepressant efficacy was the change from baseline in the mean MADRS total score during the 24 weeks of double-blind treatment, using a repeated measures analysis of variance to compare the treatment groups over all assessment points simultaneously. RESULTS: Based on the primary parameter, escitalopram was at least as efficacious as citalopram. Based on the prospectively defined secondary parameter, mean change from baseline in the CGI-S score, escitalopram was statistically significantly superior to citalopram at Week 24. The importance of long-term treatment could be demonstrated, in that more than half (55% and 51%) of the patients who had not responded by Week 8 achieved remission by Week 24. Both escitalopram and citalopram were safe and well tolerated in acute and long-term treatment, and the overall adverse event profiles for the two drugs were similar. For the intent-to-treat population, there were statistically significantly fewer withdrawals in the escitalopram group than in the citalopram group, particularly after Week 8. CONCLUSION: Patients with MDD responded well to long-term treatment with either escitalopram or citalopram. This study demonstrated the importance of extending treatment of depression beyond 8 weeks.