Increased vascularity was found in the endometrial functionalis in uteri of women wearing intrauterine contraceptive devices (IUDs) as compared to uteri of women without IUDs (control subjects). Vessel concentration was highest in endometrial tissue adjacent to that tissue which was depressed by the IUD. In control tissues there was a significant variation in vascularity according to geographic location in the following order of magnitude: fundus greater than corpus greater than cornua greater than isthmus. No significant variation was found, however, among different phases of the ovarian cycle in either control or IUD cases. Increased endometrial vascularity could be a reaction to vessel damage caused by the IUD and for several reasons may contribute to IUD-induced endometrial bleeding.