Fifty-six patients suffering from primary dysmenorrhoea were treated in a single-blind study for 3 days during each of three menstrual periods. Treatment, allocated randomly, was either 550 mg naproxen sodium or 400 mg ibuprofen initially followed by either 275 mg naproxen sodium or 400 mg ibuprofen four times daily. Assessment of dysmenorrhoea was made as soon as possible after each episode. The severity was categorized as none, mild, moderate, severe or very severe. The degree of symptom control was also graded as excellent, moderate, mild, none or symptoms worse. Effect on normal activities were recorded. Both groups responded well to treatment and comparisons between the treatment groups revealed a statistically significant difference in one aspect only; patients in the naproxen sodium group showed a significant improvement in limitation of activities (P = 0.05). Seventy-four per cent of patients in the naproxen sodium group and 61 per cent in the ibuprofen group found good or excellent symptom relief. Side-effects were similar in both groups, with several patients reporting a decrease in menstrual flow. Two patients in the ibuprofen group reported nausea, as did four patients in the naprofen sodium group, one of whom withdrew. Both treatments, therefore, were effective in relieving the symptoms of dysmenorrhoea and were well tolerated.