Endometrial epithelial metaplasia refers to the replacement of the normal endometrial glandular epithelium by cells that are either not encountered in the normal endometrium or, if present, are usually inconspicuous elements. Because these cells appear unusual or "atypical" and because they may line architecturally complex glands, this benign process is frequently confused with adenocarcinoma. This report concerns the clinical and light-microscopic findings in 89 patients whose endometria demonstrated some form of metaplasia. Most of these metaplastic changes could be placed in one of the following seven categories: 1) morules and squamous metaplasia; 2) syncytial papillary metaplasia; 3) ciliated cell metaplasia ("tubal" metaplasia); 4) eosinophilic metaplasia; 5) mucinous metaplasia; 6) hobnail metaplasia; or 7) clear cell metaplasia. The defining characteristics of each of these groups and their differential diagnoses are discussed. The majority of women whose endometria demonstrated metaplastic transformation were postmenopausal, and most had received some form of oral estrogen replacement therapy within 3 months of the time of curettage or endometrial biopsy.