The aim of this exploratory study was to systematically assess the potential effectiveness and tolerability of quetiapine, an atypical antipsychotic, for the treatment of patients with fibromyalgia. This was a unicentre, open-label study conducted in thirty-five outpatients, 18 years or older, who met the ACR criteria for fibromyalgia and who had not satisfactorily responded to their previous fibromyalgia treatment. Quetiapine, flexibly dosed (25-100 mg/day), was added to their original treatment regimen for 12 weeks. The primary outcome measure was the mean change from baseline to endpoint in the Fibromyalgia Impact Questionnaire (FIQ) total score. Secondary efficacy measures included mean changes from baseline to endpoint in the scores of the Clinical Global Impression (CGI) of Severity scale, Pittsburgh Sleep Quality Index (PSQI), Beck Depression Inventory (BDI), State-Trait Anxiety Inventory (STAI), 12-Item Short Form Health Survey (SF-12), and individual items of the FIQ. Thirty (85.7%) patients (mean age 47+/-7.9, 93.3% females) had a postbaseline evaluation and constituted the intent-to-treat efficacy sample. Mean FIQ total score decreased significantly by 10.2 points from a baseline of 63.2 to 53.0 at study endpoint (p<0.001). A statistically significant reduction was observed in FIQ stiffness and FIQ fatigue subscores but not in FIQ pain subscore. Large effect sizes were observed for the FIQ total (1.04), CGI-severity (1.00) and PSQI (1.07), while moderate effect sizes (i.e.> or =0.50) were encountered in the FIQ fatigue, FIQ stiffness and SF-12 mental component summary. Quetiapine was safely administered and well tolerated. Despite the lack of effect on pain, the significant and relevant improvement in overall efficacy measures and quality of life suggests that quetiapine may be a valuable drug for treatment of patients with fibromyalgia that should be further tested in double-blind, placebo-controlled trials.