The main objective of this study was to evaluate efficacy and tolerability of once-daily extended release quetiapine fumarate (quetiapine XR) monotherapy in generalized anxiety disorder (GAD). This was a 8 week randomized, 2-week follow-up, double-blind, placebo-controlled, and active-controlled study. Patients were randomized to quetiapine XR 150 (n=219) or 300 mg/day (n=207); escitalopram, 10 mg/day (n=213); or placebo (n=215). The primary endpoint was the change from randomization at week 8 in Hamilton Anxiety Rating (HAM-A) total score. Week 8 mean HAM-A total score was significantly reduced from randomization with quetiapine XR 150 mg/day (−13.9, P<0.001), 300 mg/day (−12.3, P<0.05) and escitalopram (−12.3, P<0.05) versus placebo (−10.7); significant improvements with quetiapine XR (150 and 300 mg/day) versus placebo (P<0.001) were also shown at day 4. At week 8, significant improvements versus placebo were observed in HAM-A psychic [quetiapine XR (both doses) and escitalopram] and somatic (quetiapine XR 150 mg/day and escitalopram) cluster scores and HAM-A response and remission rates (quetiapine XR 150 mg/day). Most common adverse events were dry mouth, somnolence and sedation (quetiapine XR), headache, and nausea (escitalopram). In patients with GAD, quetiapine XR (150 and 300 mg/day) demonstrated significant efficacy at week 8 with symptom improvement as early as day 4. We concluded that quetiapine XR safety and tolerability results were consistent with the known profile of quetiapine.