Eleven endometrial adenoacenthomas were studied ultrastructurally. Special emphasis was placed on patterns of squamous metaplasia and the appearance of the basal laminae. Tonofibrils, irregular keratohyaline, abundant desmosomes, and foci of frank keratinization were observed. However, the granule population was small, with relatively scanty keratohyaline and no definite membrane coating granules. These features resemble squamous metaplasia observed in various epithelia; experimental models utilizing estrogens also resulted in similar changes. This observation appears pertinent in view of the possible etiologic role played by high estrogen levels in the development of human endometrial carcinoma. Most tumor cell clusters were surrounded by a well-defined basal lamina; focal basal lamina reduplication was also observed. These observations suggest a high level of differentiation in respect to synthetic capabilities and cell surface recognition. They correlate well with the relatively favorable prognosis generally observed in endometrial adenoacanthomas. However, these conclusions are tempered by the finding of occasionally absent or defective basal lamine, indicative of partial dedifferentiation of some neoplastic cells.