The efficacy of indomethacin, a prostaglandin synthetase inhibitor, in severe dysmenorrhoea was established in a double-blind crossover study using aspirin and placebo as the control drugs. Forty-seven female undergraduates were treated twice with each of the three substances during six consecutive menstrual cycles. Good or moderate relief was achieved in 71% of the cycles treated with indomethacin, in 40% of those treated with aspirin and in 21% of those treated with the placebo. Dizziness and drowsiness were cited by 14 patients (30%) as side-effects of indomethacin, none of these patients discontinued the therapy because all obtained good or moderate relief from dysmenorrhoea. Indomethacin proved to be a valuable agent, and significantly better than aspirin in the treatment of dysmenorrhoea. It allowed many dysmenorrhoeic women to carry out their normal activities and work during the menstrual period.