We examined the incidence of endometrial cancer in a large prepaid group practice in the Seattle area. From July, 1975, to July, 1977, there was a sharp downward trend in the incidence of endometrial cancer that paralleled a substantial reduction in prescriptions for replacement estrogens. Incidence rates were estimated for estrogen users and nonusers among women 50 to 64 years of age with intact uteri; current long-term users had an annual risk for endometrial cancer between 1 and 3 per cent, whereas nonusers had a risk less than 1/10th as great. These incidence rates remained fairly constant over time among users and nonusers; the drop in overall incidence soon after estrogen use declined suggests that the increased risk associated with estrogens falls quickly after discontinuation. The reduction in incidence of endometrial cancer in this group practice was part of a general decline in the United States after 1975.