In contrast to several retrospective studies reporting an increased risk of endometrial cancer during the mid-1970s, especially in estrogens.gen-treated postmenopausal women, the number of cancers at Wilford Hall USAF Medical Center has steadily declined despite continued estrogen use. In a 4-year study from 1975 to 1978, there were 17 adenocarcinomas of the endometrium during 10,872 patient-years of observation, for an overall annual incidence of 156.4:100,000 women. The highest incidence of endometrial cancer (359.1:100,000) was found in those women using estrogens alone. The lowest incidence of cancer was observed in the estrogen-progestogen users (56.4:100,000) and was significantly lower (P less than .01) than that found in the estrogen users. The incidence of corpus malignancy in the estrogen-progestogen users was also significantly lower (P less than .05) than that observed in the untreated women (248.3:1000,000). The progestogen challenge test has been devised to identify postmenopausal women at greatest risk for adenocarcinoma of the endometrium. It is concluded that the use of this test will reduce the risk of endometrial cancer in both estrogen-treated postmenopausal women and women with increased endogenous estrogens.