The ultrastructure of the vascular supply of the endometrium in uteri containing inert and copper bearing intrauterine contraceptive devices (IUCDs) has been compared with that in uteri removed at comparable stages of the menstrual cycle for dysfunctional uterine bleeding. In the endometrium from all uteri containing inert and copper IUCDs, stromal capillaries below the devices showed extensive microthrombosis with platelet and fibrin aggregations filling gaps in the endothelial linings of the vessels. Considerable stromal haemorrhage and erosion of surface epithelium was associated with this vascular pathology. In the uteri removed for dysfunctional bleeding, microthrombosis was a rare occurrence. In the stromal vessels of the endometrium not in contact with the devices, the surface epithelium was usually intact and the underlying stromal capillaries often showed breaks in the endothelial lining but without a haemostatic plug. Leucocyte migration into the stroma was more evident in association with the copper IUCDs than the other devices. The ultrastructural changes in the microvasculature of the endometrium associated with IUCDs did not appear to be related to a stage of the menstrual cycle. The extent and type of microvascular changes appeared to be related to the proximity and surface area of the IUCD and to the presence or absence of copper in the device.