Although electron microscopy has been of limited value in detecting malignant neoplasms, some neoplasms do exhibit characteristic ultrastructural features. Intensive study of these features may offer insight into the etiology and activity of such tumors. In an attempt to characterize the ultra-structure of endometrial adenocarcinoma, tissue was examined with the electron microscope and compared with normal postmenopausal endometrium. Endometria from biopsy, hysterectomy, or curettage were processed routinely for light and electron microscopy. Several ultrastructural features of the adenocarcinoma were common both to previous descriptions of endometrium of the postovulatory phase and to the normal postmenopausal endometrium, described here, viz, atypical mitochondrial forms and cell surface modifications. Ribosomes were abundant in adenocarcinoma of the endometrium, normal postmenopausal endometrium, and normal cyclic endometrium in the preovulatory phase.