To evaluate the efficacy and tolerability of extended release quetiapine fumarate (quetiapine XR) in a 6-week, double-blind, randomized study. Patients with a DSM-IV diagnosis of acute schizophrenia were randomly assigned to fixed-dose quetiapine XR 400, 600, or 800 mg/day (once daily in the evening), quetiapine immediate release (IR) 400 mg/day (200 mg twice daily), or placebo. Dual-matched placebo was used to maintain blinding. Quetiapine XR target doses were reached by day 2 (400 and 600 mg) and day 3 (800 mg). The primary endpoint was least squares mean change from baseline to week 6 in Positive and Negative Syndrome Scale (PANSS) total score. PANSS response rate (percentage of patients with > or = 30% reduction in total score), Clinical Global Impressions-Improvement scale (CGI-I) response rate (percentage of patients with score < or = 3), change in CGI-Severity of Illness (CGI-S), and adverse events (AEs) were also assessed. The study was conducted from November 2004 to December 2005. 588 patients were enrolled and 446 (76%) completed the study. Improvement in PANSS total score at week 6 was significant versus placebo (-18.8) in all groups: -24.8 (p = .03), -30.9 (p < .001), and -31.3 (p < .001) for quetiapine XR 400, 600, and 800 mg, respectively, and -26.6 (p = .004) for quetiapine IR. There were also statistically significant differences in PANSS and CGI-I response rates for all active treatments versus placebo (all p < .05). The most common AEs in all quetiapine groups were somnolence and dizziness; there were no unexpected AEs with quetiapine XR. Incidence of AEs potentially related to extrapyramidal symptoms was similar to placebo. Once-daily quetiapine XR (400-800 mg/day) was effective versus placebo in patients with acute schizophrenia. Treatment, including rapid dose escalation, was well tolerated, with a therapeutically effective dose reached by day 2. ClinicalTrials.gov identifier NCT00206115.