Endometrial hyperplasia is a noninvasive proliferation of the endometrium that results in a morphologic pattern of glands with irregular shapes and varying size. This disorder results from sustained, unopposed estrogen stimulation and presents clinically as abnormal uterine bleeding. Sometimes hyperplasia is encountered incidentally in a biopsy performed for other reasons, such as infertility workup prior to or during hormone replacement therapy. Hyperplasia can mimic a wide variety of normal physiologic changes, artifacts resulting from tissue sampling and processing, benign organic disorders, and well-differentiated adenocarcinoma. Because management of these conditions and the different forms of hyperplasia can range from no treatment to hysterectomy, correct diagnosis is essential.