Prospectively planned pooled analysis evaluating the efficacy of quetiapine extended release (XR) monotherapy in major depressive disorder (MDD). Data were pooled from two 6-week, randomized, double-blind, placebo-controlled studies of quetiapine XR in outpatients with MDD. The primary endpoint was Montgomery-Åsberg Depression Rating Scale (MADRS) total score change from randomization at week 6. Other evaluations were MADRS response/remission, Hamilton Rating Scale for Anxiety, and subgroup analyses. A total of 968 patients were randomized to quetiapine XR, 150 mg/day (n=315), 300 mg/day (n=323), or placebo (n=330). The mean MADRS total score reductions from randomization were significant at week 6 with quetiapine XR, 150 mg/day (−14.7; P<0.001) and 300 mg/day (−14.7; P<0.001) versus placebo (−11.1), with significant reductions versus placebo from week 1 onward. Response rates (week 6): 52.7% (P<0.001) quetiapine XR 150 mg/day and 49.5% (P<0.001) quetiapine XR 300 mg/day versus placebo (33.0%). MADRS remission (score ≤8; week 6): 23.5% (P=0.208) quetiapine XR 150 mg/day and 28.8% (P<0.01) quetiapine XR 300 mg/day versus placebo (19.4%). Quetiapine XR (both doses) significantly improved eight of 10 MADRS items versus placebo at week 6. The therapeutic effect of quetiapine XR was neither limited to nor driven by factors such as sex, age, or severity of depression. In patients with MDD, quetiapine XR (150 and 300 mg/day) monotherapy reduced depressive symptoms, with significant improvements compared with placebo from week 1 onward.