OBJECTIVE: This randomised, double-blind, fixed-dose study evaluated the efficacy of escitalopram and paroxetine in the long-term treatment of severely depressed patients with major depressive disorder (MDD). RESEARCH DESIGN AND METHODS: Patients with a primary diagnosis of MDD and baseline Montgomery-Asberg Depression Rating Scale (MADRS) >or= 30 were randomised to 24 weeks of double-blind treatment with fixed doses of either escitalopram (20 mg) (n = 232) or paroxetine (40 mg) (n = 227). The primary analysis of efficacy was an analysis of covariance (ANCOVA) of change from baseline to endpoint (Week 24) in MADRS total score (last observation carried forward, LOCF). MAIN OUTCOME MEASURES; RESULTS: At endpoint (24 weeks), the mean change from baseline in MADRS total score was -25.2 for patients treated with escitalopram (n = 228) and -23.1 for patients with paroxetine (n = 223), resulting in a difference of 2.1 points (p < 0.05). The difference in the change in the MADRS total score (LOCF) was significantly in favour of escitalopram from Week 8 onwards. The proportion of remitters (MADRS <or= 12) after 24 weeks was 75% for escitalopram and 67% for paroxetine (p < 0.05). The results on the primary efficacy scale were supported by significantly greater differences in favour of escitalopram on the Hamilton Anxiety, Hamilton Depression and Clinical Global Impression-Improvement and -Severity scales. For very severely depressed patients (baseline MADRS >or= 35), there was a difference of 3.4 points at endpoint in the MADRS total score in favour of escitalopram (p < 0.05). The overall withdrawal rate for patients treated with escitalopram (19%) was significantly lower than with paroxetine (32%) (p < 0.01). The withdrawal rate due to adverse events was significantly lower for escitalopram (8%) compared to paroxetine (16%) (p < 0.05). There were no significant differences in the incidence of individual adverse events during treatment. CONCLUSION: Escitalopram is significantly more effective than paroxetine in the long-term treatment of severely depressed patients.