The calcium antagonist, nifedipine, was used to identify patients with primary dysmenorrhea caused by myometrial hyperactivity. Twelve patients with severe primary dysmenorrhea received an oral loading dose of 30 mg nifedipine on the first day of menstruation. Nine patients reported prompt relief of the menstrual cramps (within 15-60 min). In 3 patients no pain relief was obtained. In 2 of these subjects, subsequent laparoscopy revealed obvious signs of endometriosis and previous pelvic inflammatory disease as the cause of these patients pain. Moreover, intrauterine pressure recording with microtransducers displayed normal uterine activity in these patients. It is concluded that, due to its prominent tocolytic effect, nifedipine can be used as a simple pharmacologic test to identify patients suffering from severe primary dysmenorrhea. In addition it indicates a subsequent way to treat the disorder.