Resected uteri from a series of 48 consecutive patients with atypical endometrial hyperplasia ("carcinoma in situ") identified in curettings were studied to determine the frequency of associated adenocarcinoma. Thirty-nine of the women had received estrogen or oral contraceptives. Twelve uteri (25%) contained small superficial foci interpreted as well differentiated adenocarcinoma. Consideration of this observation together with the absence of adenocarcinoma in the extensive experience reported in other studies of women with atypical hyperplasia treated with progestogens alone shows a deficit of patients with progression to invasive carcinoma. We reconcile this apparent conflict with the hypothesis that lesions classified as focal well-differentiated endometrial adenocarcinoma occurring with atypical hyperplasia in perimenopausal women exposed to estrogen are common and are consistently reversed by progestogen therapy.