Sixty-three women, aged 18 to 39 years, with primary dysmenorrhea received 25 mg, 50 mg, or 75 mg of ketoprofen, 500 mg of naproxen, or placebo as a first dose at the onset of moderate or severe pain. Each patient received three treatments and each treatment was tested in 36 patients. Mean pain relief scores (on a five-point scale) indicated a significant analgesic response for all active treatments; superiority over placebo was shown by ketoprofen 50 mg for six hours, by ketoprofen 75 mg for five hours, by ketoprofen 25 mg for four hours, and by naproxen for four hours. The onset of pain relief and peak relief were reached faster and pain relief lasted longer after 75 mg and 50 mg of ketoprofen than after 25 mg of ketoprofen or 500 mg of naproxen, which in turn were superior to placebo. Treatment was rated good to excellent by 20 patients after 25 mg of ketoprofen, by 26 after 50 mg, and by 28 after 75 mg, and by 22 after naproxen and 11 after placebo. The incidence of side effects was similar in the ketoprofen-treated and naproxen-treated patients.