Recent reports suggest that atypical endometrial hyperplasia diagnosed by biopsy or curettage is accompanied by a higher than expected risk of coexistent invasive cancer. In order to test this hypothesis we reviewed the pathology and clinical history of all patients at our institution who underwent hysterectomy for endometrial hyperplasia with or without cytologic atypia. We found 24 patients of 45 with a preoperative diagnosis of hyperplasia with cytologic atypia, and 21 with simple or complex hyperplasia without atypia. No cancers were found at surgery in the latter group nor were any significant historical differences found between the two groups. Of the patients with atypia, 12/24 (50%) had an endometrial carcinoma and nine patients (37.5%) were stage IB or greater. This is a significantly greater risk than previously reported in the literature. Endometrial hyperplasia with cytologic atypia may carry a higher risk of coexistent invasive endometrial carcinoma than previously believed. Methods to identify those patients at highest risk should be determined.