The spiral arterioles in normal proliferative and secretory endometrium were compared with those of hyperplastic and contraceptive-affected endometriums. No smooth muscle or endothelial cell hyperplasia was present in 100 samples of proliferative and secretory endometrium. Two thirds of 100 hyperplastic endometriums showed smooth muscle hyperplasia of the spinal arterioles; one-third of the specimens also had endothelial proliferation of the vessels. The same changes were observed in one-third of 78 endometrial biopsies from patients taking oral contraceptives, both combination and sequential. Estrogen may be the causative agent in both types of altered endometriums.