The efficacy and tolerability of extended-release quetiapine fumarate (quetiapine XR) once-daily monotherapy in generalized anxiety disorder (GAD) was assessed. This multicentre, double-blind, randomized, placebo- and active-controlled, phase III trial consisted of a 1- to 4-wk enrolment/wash-out period and a 10-wk (8-wk active treatment, 2-wk post-treatment drug-discontinuation) study period; 873 patients were randomized to 50 mg or 150 mg quetiapine XR, 20 mg paroxetine, or placebo. Primary endpoint was change from randomization at week 8 in Hamilton Rating Scale for Anxiety (HAMA) total score. At week 8, all active agents produced significant improvements in HAMA total and psychic subscale scores vs. placebo; HAMA somatic subscale scores were significantly reduced only by 150 mg quetiapine XR. Significant separation from placebo (-2.90) in HAMA total score was observed at day 4 for 50 mg quetiapine XR (-4.43, p<0.001) and 150 mg quetiapine XR (-3.86, p<0.05), but not for paroxetine (-2.69). Remission (HAMA total score 7) rates at week 8 were significantly higher for 150 mg quetiapine XR (42.6%, p<0.01) and paroxetine (38.8%, p<0.05) vs. placebo (27.2%). The most common adverse events (AEs) were dry mouth, somnolence, fatigue, dizziness, and headache, for quetiapine XR, and nausea, headache, dizziness for paroxetine. A lower proportion of patients reported sexual dysfunction with quetiapine XR [0.9% (50 mg), 1.8% (150 mg)] than with placebo (2.3%) or paroxetine (7.4%). The incidence of AEs potentially related to extrapyramidal symptoms was: quetiapine XR: 50 mg, 6.8%, 150 mg, 5.0%; placebo, 1.8%; and paroxetine, 8.4%. Once-daily quetiapine XR is an effective and generally well-tolerated treatment for patients with GAD, with symptom improvement seen as early as day 4.