To compare the effect of naproxen on idiopathic and myoma-induced menorrhagia, 11 women with myomatosus uterus and 14 women with idiopathic menorrhagia (menstrual blood loss greater than 80 mL) were treated in a double-blind trial with placebo or naproxen during four consecutive menstruations. Placebo had no effect on menstrual blood loss. Naproxen (500 to 1000 mg daily for five days) reduced menstrual blood loss by 35.7% in women with idiopathic menorrhagia, but it had no consistent effect on myoma-induced menorrhagia. No side effects occurred during naproxen use. Thus, naproxen may prove a suitable treatment for idiopathic but not for myoma-induced menorrhagia.