The International Society of Gynecological Pathologists recently agreed on a classification of endometrial hyperplasia into two main groups; hyperplasias with and without atypia. The lesions were further subdivided into simple and complex hyperplasia. These guidelines were subsequently adopted by the World Health Organization. The disease is a result of oestrogen/gestagen imbalance with oestrogen overexpression. The most important prognostic factor is cellular atypia. Progress to invasive cancer is seen in about 20% of the patients with atypical hyperplasia, and most frequently occurs in postmenopausal women. The treatment of endometrial hyperplasia depends on histologic type, patients' age and whether the hyperplasia is a result of endogenous or exogenous oestrogen overexpression. The risk for progression to invasive cancer is minimal in oestrogen treated patients with simple or complex hyperplasia without atypia. Women under 40 years of age in this group can safely be treated with gestagens. In postmenopausal women with simple or complex hyperplasia with atypia, the recommended treatment is surgery including removal of the uterus and the ovaries.