Endometrial hyperplasia is a condition of excessive proliferation of the cells of the endometrium, or inner lining of the uterus.

[4] In 2014, the WHO updated the classification system and removed the distinction between simple or complex hyperplasia, instead only on presence or absence of atypia.

A workup for endometrial disease may be prompted by abnormal uterine bleeding, or the presence of atypical glandular cells on a pap smear.

Further it states the need for population based studies including both non-atypical and atypical hyperplasia to accurately estimate the risk of progression to cancer.

[11][12] Given this, the aforementioned 28% atypia progression rate may be an underestimate, and the true number may closer to the 42.5% part of the study's remarkably wide confidence interval.