The clinicopathologic features of nine cases of papillary proliferation of the endometrium devoid of malignant nuclear features were studied. The patients ranged in age from 33 to 71 years (median 57 years). All were postmenopausal, except the youngest. The most common symptom was postmenopausal bleeding. Two patients were receiving hormonal replacement therapy and two were taking megestrol acetate. Two lesions were incidental findings in a hysterectomy specimen. Seven were diagnosed in endometrial biopsy or curettage specimens. In six cases (67%) the lesion involved an endometrial polyp. In all cases the papillae had fibrovascular stromal cores and variable degrees of branching. Two architectural patterns were found. A simple papillary pattern with involvement of only a few glands and little epithelial proliferation occurred in five cases, including three that were entirely intracystic. A complex papillary pattern with more extensive involvement of endometrial glands, a greater degree of branching of the papillae, and cellular tufting occurred in four cases. One or more metaplastic epithelial changes occurred in all cases, including endocervical-type mucinous metaplasia in nine cases (90%), eosinophilic cell change in eight (89%), ciliated cell change in seven (70%), focal squamous metaplasia in two cases (22%), and hobnail cell change in two (22%). Mitotic figures were found in three cases. In four lesions (44%), all with a complex papillary pattern, the proliferating cells had mild nuclear atypia. Three of these patients underwent hysterectomy within 5 months. Simple nonpapillary hyperplasia and two endometrial polyps were found in one patient, complex nonpapillary hyperplasia in one, and atrophic endometrium in the other. Two patients had additional endometrial samplings within 4 months that contained small residual simple papillary lesions. One of these had another biopsy at 16 months that showed only atrophy. One patient had no subsequent diagnostic or therapeutic procedures. One patient was a recent case. Of the three patients with intact uteri and appreciable follow-up, all were alive and well at 14, 96, and 102 months, respectively. We conclude that these papillary proliferations are a form of hyperplasia that is closely associated with endometrial epithelial metaplasia. Polypectomy and/or curettage may be effective in removing them because they often are localized lesions. Although all of our patients had an uneventful outcome, the number of cases is small. Our findings question the validity of diagnosing endometrial lesions as well-differentiated carcinoma solely because of a complex papillary architectural pattern.