Premenstrual dysphoric disorder (PMDD), as defined in DSM-IV, is a mood disorder. One of the leading theories for the pathogenesis of mood disorders is dysfunction of the serotonergic system. An increasing database suggests that serotonergic dysfunction also characterises PMDD. Evidence that treatments which enhance serotonergic function are beneficial in reducing the symptoms of PMDD support this hypothesis. Indeed, most of the evidence from baseline studies suggests predominantly a serotonergic rather than a noradrenergic or dopaminergic dysfunction. Challenge studies further support this hypothesis. These findings of neurotransmitter dysfunction are more consistent than those of other neuroendocrine abnormalities for example. Based on treatment studies, a selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor, fluoxetine, has been approved for use in PMDD by the US Food and Drug Administration.