A double-blind crossover study was carried out to evaluate the therapeutic efficacy of nimesulide and its effects on uterine activity, menstrual fluid prostaglandin F, and pain in women suffering from primary dysmenorrhea. Twenty-three women entered the clinical pharmacologic study. Intrauterine pressure was monitored with a microballoon-tipped catheter on the first day of menstruation. During the maximal pain period (based on monitoring in six patients), nimesulide significantly decreased intrauterine pressure; the measure of pain relief was consistent with decrease of uterine activity. In another six patients, the registration of intrauterine pressure during the submaximal pain period demonstrated that both in the nimesulide- and placebo-treated cycles, the uterine activity was at a lower mean level than that registered during maximal pain. Furthermore, when two 100-mg oral doses of nimesulide were administered to 11 dysmenorrheic women, in double-blind, crossover conditions with placebo as a blank reference, it brought about a reduction of menstrual fluid prostaglandin F2 levels from 382 to 94 ng/mL, (P less than .001). Fourteen women entered a four-cycle, double-blind, crossover therapeutic trial. Each patient was randomly assigned to one of two treatment sequences with nimesulide 200 mg/d PO or placebo. The therapy was judged very effective or good in 22 of 28 cycles treated with nimesulide compared with nine of 27 cycles treated with placebo (P less than .01). The amount of bleeding during the treated cycles did not change, and there were no complaints of untoward signs or symptoms related to the therapies.