MSH: Benign proliferation of the ENDOMETRIUM in the UTERUS. Endometrial hyperplasia is classified by its cytology and glandular tissue. There are simple, complex (adenomatous without atypia), and atypical hyperplasia representing also the ascending risk of becoming malignant.,NCI: An abnormal overgrowth of the endometrium (the layer of cells that lines the uterus). There are four types of endometrial hyperplasia: simple endometrial hyperplasia, complex endometrial hyperplasia, simple endometrial hyperplasia with atypia, and complex endometrial hyperplasia with atypia. These differ in terms of how abnormal the cells are and how likely it is that the condition will become cancer.,NCI: A proliferation of the endometrial cells resulting in glandular enlargement and budding. The proliferation may or may not be associated with atypia of the endometrial cells. When the hyperplastic changes are excessive, there is formation of complex epithelial structures (complex endometrial hyperplasia).