Twenty-one women with intrauterine contraceptive devices (IUCD) and severe dysmenorrhea were studied. All the women who participated in the study had primary dysmenorrhea of varying intensities. The insertion of IUCD increased the intensity of dysmenorrheic pain. The effect of naproxen (Naprosyn) on pain alleviation was studied in a double-blind cross-over trial using naproxen and placebo. The effect of naproxen was significantly better than that of placebo (P less than 0.01). No severe side effects occurred during the treatment. There was no difference in the duration and amount of the menstrual blood flow during naproxen treatment compared to placebo according to the women's own judgement.