BACKGROUND: Current guidelines for antidepressant use recommend 4 to 6 months of continuation treatment to prevent relapse of depression following symptom resolution. This study evaluates the efficacy and safety of continuation escitalopram treatment. METHOD: Outpatients diagnosed with DSM-IV major depressive disorder (male or female, aged 18 to 81 years) who had completed 8 weeks of randomized double-blind treatment with escitalopram, citalopram, or placebo entered an 8-week flexible-dose, open-label phase in which all patients received escitalopram (10-20 mg/day). This study was initiated November 3, 1999, and completed April 5, 2001. Patients who met responder criteria (score of < or = 12 on the Montgomery-Asberg Depression Rating Scale [MADRS]) were randomly assigned in a 2:1 ratio to escitalopram (at the dose each patient was receiving at the end of the open-label phase) or placebo for 36 weeks of double-blind treatment. The primary efficacy variable was time to depression relapse (defined as MADRS score > or = 22 or discontinuation due to an insufficient therapeutic response) from the start of the double-blind treatment phase. RESULTS: A total of 502 patients received open-label escitalopram treatment and had at least 1 MADRS assessment. A total of 274 evaluable subjects entered the double-blind treatment phase; 93 received placebo and 181 received escitalopram. Time to depression relapse was significantly longer (p =.013) and the cumulative rate of relapse was significantly lower in patients who received escitalopram (26% escitalopram vs. 40% placebo; hazard ratio = 0.56; p =.01). Escitalopram-treated subjects had significantly lower depression ratings than those of placebo-treated patients. Escitalopram continuation treatment was safe and well tolerated. Discontinuation rates due to adverse events were 7% for the placebo group and 4% for the escitalopram-treated group. CONCLUSION: Continuation treatment with escitalopram is effective in preventing relapse into an episode of major depressive disorder.