Primary dysmenorrhea is a difficult entity to treat, and therapy is usually directed at relieving symptoms. There is some indication that this disorder is caused by an increase in prostaglandin F2alpha. Therefore, logically the treatment may include antiprostaglandin agents. We have studied 32 women with the diagnosis of primary dysmenorrhea in a randomized double-blind fashion using a placebo and indomethacin. Both agents were taken three times a day over four cycles, and therapy was begun two days before the usual onset of pelvic pain. Only two of 16 patients in the placebo group were significantly improved in the four-month treatment cycles while all 16 in the treatment group showed some improvement, 11 having cessation of pain. In the six months following the study period, all patients were given indomethacin. The original treatment group did not change significantly. However, all in the placebo group when switched to indomethacin had some relief, 12 of the 16 showing complete cessation of pain. Gastric irritation was the main side effect and was present in 18% of the treatment group and 12% in the placebo group. Indomethacin appears to effectively relieve primary dysmenorrhea and does not appear to be associated with a high incidence of side effects.