The various forms of endometrial hyperplasia are classified and a description given of their pathology and of their inter-relationships. The nosological status of endometrial adenocarcinoma in situ and the histological differentiation between hyperplasia with severe cellular atypia and endometrial adenocarcinoma are discussed. The aetiology and pathogenesis of the various endometrial hyperplasias are considered and grounds are given for the belief that glandular hyperplasia with cellular atypia is a form of intraendometrial neoplasia. A new terminology, based on the biological nature of the various endometrial abnormalities, is proposed and a plea is made for the introduction of the term 'intraendometrial neoplasia' to encompass all forms of endometrial glandular hyperplasia with cellular atypia and intraendometrial adenocarcinoma.