The efficacy of quetiapine XR was investigated in patients with major depressive disorder and differing levels of baseline severity. Pooled data from four placebo-controlled monotherapy studies of quetiapine XR (50–300 mg/day) were analyzed. Post-hoc analyses were carried out to assess change from baseline in the Montgomery Åsberg Depression Rating Scale (MADRS) total score at endpoint (week 6 or 8) to week 1, and response (≥50% reduction in MADRS total score) and remission (MADRS total score≤10) rates at endpoint for all patients and six baseline severity cohorts (MADRS total score ≥24, ≥26, ≥28, ≥30, ≥32, and ≥34). In total, 1752 patients (all patients) were evaluated (MADRS score at baseline: ≥24, n=1601; ≥26, n=1467; ≥28, n=1269; ≥30, n=1038; ≥32, n=745; and ≥34, n=500). At endpoint, quetiapine XR reduced MADRS total score in all patients (P<0.001) and each severity cohort (≥24, ≥26, ≥28, ≥30, and ≥32, P<0.001; ≥34, P<0.01) versus placebo. Quetiapine XR also improved MADRS total score at week 1, response rates for each severity cohort, and remission rates in five out of six severity cohorts, versus placebo. Quetiapine XR monotherapy showed antidepressant effects in patients with major depressive disorder across different levels of baseline severity.