Since its first description more than four decades ago, the atypical endometrial change associated with chorionic tissue effect has been widely confirmed in the literature. However, errors and inaccuracies in text books and other publications often occur. This review clarifies some of these misconceptions and presents a summary of new data on the histologic and immunohistochemical characteristics of the change. A brief discussion of the pathogenesis and biologic significance of the alteration is included. © 2002 Lippincott Williams & Wilkins, Inc.