The objective of this study was to investigate whether quetiapine, when compared with placebo, can speed the onset of action and improve the quality of response to fluoxetine treatment in patients suffering from major depressive disorder. A total of 114 patients with major depressive disorder were enrolled in an 8-week treatment study. Patients were initiated on a course of fluoxetine treatment and randomized to quetiapine or placebo. Quetiapine was flexibly dosed starting at 25 mg to a maximum of 100 mg daily. Mixed-effects regression showed that quetiapine plus fluoxetine did not achieve 50% reduction in the Montgomery-Asberg Depression Rating Scale score or improvement in Hamilton Anxiety Scale, Clinical Global Improvement (CGI)-Severity, and CGI-Improvement scores sooner than the fluoxetine plus placebo group; however both groups improved in all scores over time. Mixed-effects linear regression of insomnia scores showed that the quetiapine plus fluoxetine group improved significantly more rapidly compared with the fluoxetine plus placebo group. The study indicates that quetiapine plus fluoxetine did not achieve a reduction in the Montgomery-Asberg Depression Rating Scale score or improvement in Hamilton Anxiety Scale or CGI scores from baseline sooner than the fluoxetine plus placebo group. The combination of quetiapine and fluoxetine, however, improved sleep over fluoxetine alone over the first few weeks of treatment.