Premenstrual dysphoric disorder (PMDD), a more severe form of premenstrual syndrome (PMS), afflicts 5-8% of reproductive age women and results in significant functional impairment. We conducted a double-blind, placebo-controlled trial of adjunctive quetiapine in patients with PMS/PMDD who had inadequate response to selective serotonin reuptake inhibitor/serotonin-norepinephrine reuptake inhibitor therapy for their symptoms. A PMS/PMDD diagnosis was confirmed by 2-month prospective diagnostic assessment of PMS/PMDD using the Prospective Record of the Impact and Severity of Premenstrual Symptoms (PRISM) calendar. Women were randomized equally to receive quetiapine sustained-release (SR) or placebo (25-mg starting dose) during the luteal phase for 3 months. Outcome variables included the Hamilton Depression and Anxiety Scales, Clinical Global Impression Scale, and PRISM. Twenty women were enrolled in the treatment phase. Although the study was underpowered, greater reductions in luteal phase mood ratings were observed in the quetiapine group on the 17-item Hamilton Depression Rating Scale, Clinical Global Impression improvement rating, and PRISM daily score. The quetiapine group showed most improvement in symptoms of mood lability, anxiety, and irritability. This small double-blind study suggests that adjunctive treatment with quetiapine SR may be a useful addition to selective serotonin reuptake inhibitor therapy in women with PMS/PMDD by reducing symptoms and improving quality of life.