Premenstrual tension syndrome (PMS) and rapid-cycling bipolar affective disorder have similarities of symptoms, cyclical mood swings, and putative neurotransmitter dysfunction. The possible relationship between these disorders was assessed by evaluating 25 patients with rapid-cycling disorders and 25 normal controls for PMS symptoms. Patients with rapid-cycling affective disorder had an increased tendency to have more severe forms of PMS. In addition, patients with rapid-cycling disorders and more severe forms of PMS tended to cycle more frequently. The significance of this finding and its clinical implications are discussed.