The effect of naproxen, Naprosyn, Syntex, in treatment of primary dysmenorrhea was studied in a double-blind, randomized, placebo controlled multicenter study. Nintyseven women, aged between 18--40 years, with severe dysmenorrhea, were treated with either naproxen, 48 women, or placebo, 49 women, for two consecutive menstrual cycles. No oral contraceptive was used. The patients were allowed to take supplementary analgesics 4--6 hours after the study drug was taken if adequate relief was not achieved. The recommended dose of naproxen was 1--2 tablets, 250 mg, as needed, with a maximum of 5 tablets daily. Medication was started at first sign of menstrual distress. Improvement was achieved in 70 per cent of the women in the naproxen group (good to excellent relief) but only in 30 per cent in the placebo group. This difference is statistically significant (p less than 0.001). There was much more supplementary medication used in the placebo group compared to the naproxen treated patients (p less than 0.001). Fewer patients had to stay in bed, or stay at home from work or school, in the naproxen group compared to the placebo group. Few side-effects were reported and most of them belonged to the dysmenorrhea symptomatology. No side-effects could be rement according to the patients' own judgement.