A well controlled double blind prospective trial was undertaken to determine the efficacy of mefenamic acid in the treatment of menorrhagia. Eighty patients who had unexplained cyclical menorrhagia with secretory endometrium received mefenamic acid orally in a dose of 500 mg 8 hourly or a placebo tablet thrice daily starting from day-one of the cycle for 5 days or till cessation of menses. Mefenamic acid administration caused a statistically significant reduction in the number of days of bleeding from 9.7 +/- 3.1 days before treatment to 4.1 +/- 0.6 days after treatment. It also brought about significant reduction in the amount of bleeding in terms of number of pads used per day, that is from 15.2 +/- 3.1 before treatment to 6.5 +/- 0.02 after treatment. Eighty-six percent patients in the treatment group had control of menorrhagia with the drug compared to 20% in the control group without drugs and this difference was statistically significant. Mefenamic acid proved to be a potent and efficacious agent in the control of unexplained menorrhagia.