OBJECTIVES: To investigate the efficacy of single daily doses of rofecoxib, a selective inhibitor of cyclooxygenase-2, in dysmenorrhea. METHODS: Fifty-five patients were included in this randomized, placebo-controlled, cross-over study. Patients were randomized to use placebo, naproxen sodium (550 mg), 25 and 50 mg doses of rofecoxib in various orders. Pain intensity, analgesic efficacy of drugs, total number of pills used and side effects were evaluated. RESULTS: Rofecoxib with daily single doses of 25 and 50 mg decreased the pain intensity in a manner similar to naproxen sodium. Most of the patients (85.45% and 96.46%) evaluated the analgesic efficacy of rofecoxib as 'perfectly effective'. Rofecoxib was found to be effective for pain relief both in primary and secondary dysmenorrhea. Gastrointestinal adverse effects were less than those with naproxen sodium. CONCLUSIONS: A 'one a day' dose of 25 mg rofecoxib is an effective choice with lower gastrointestinal adverse effects than naproxen sodium.