Prostaglandins have been shown to be increased in the endometrium of women and experimental animals wearing intrauterine devices (IUDs). As prostaglandins may cause increased vascularity and vascular permeability, and as certain prostaglandins (PgI2) inhibit platelet activity, the local generation of prostaglandins may contribute to endometrial bleeding. Thus, the effect of ibuprofen, a prostaglandin synthetase inhibitor, was tested by quantifying menstrual blood loss in 20 women wearing IUDs in a double-blind, 2-period crossover study. Ibuprofen produced a significant reduction in menstrual blood loss; the percentage reduction was greater in women using a Lippes Loop and who had heavier blood loss (39%) than in women using a copper device and who had lighter blood loss (25%). These findings support the contention that prostaglandin synthesis is important in the genesis of IUD-associated menorrhagia and that prostaglandin inhibitors may be useful in the therapy of this condition.