Seven samples of postmenopausal endometrium were studied by electron microscopy. Four samples were diagnosed as adenomatous hyperplasia (2 of which were atypical) and 3 as normal postmenopausal endometrium. The most striking ultrastructural features of hyperplastic endometrium were: numerous nucleoli, deep nuclear membrane infoldings, increased nucleocytoplasmic ratio, prominent and enlarged RER closely associated with mitochondria and nuclear membrane, abundant free ribosomes and marked network microfilaments. In the case of atypical adenomatous hyperplasia, the protruded intraglandular proliferating epithelial cells exhibited more atypical features than the epithelial cells of the glandular lining, suggesting a more advanced degree of anaplastic change. The characteristic features of the normal postmenopausal endometrium were: paucity and random distribution of the cytoplasmic organelles, the presence of large cytoplasmic vacuoles and short, blunt microvilli. Secretory vacuoles opening into the lumen of the gland were found in one case of cystic atrophy of a normal postmenopausal endometrium. Collagenization was found in the stroma of both groups, although predominantly in the normal postmenopausal endometrium. In 2 cases of adenomatous hyperplasia stromal cells with vacular cytoplasm were found. The significance of these findings, as related to their importance as precursor stages of endometrial cancer (in the cases of adenomatous and atypical adenomatous hyperplasia); as involutional manifestations (in the cases of normal postmenopausal endometrium); and as related to the absence of cyclic activity (in both groups) is briefly discussed.