The treatment of patients with the diagnosis atypical endometrial hyperplasia has been disputed during the last decades. The aim of the study was to evaluate the treatment of these patients and analyse the progression rate to invasive carcinoma of the endometrium. Fifty-seven patients with atypical hyperplasia were examined and treated from 1976 through 1991. The medical records were examined retrospectively and the pathology slides were revised by one pathologist in accordance with the 1975 WHO recommendations. Thirty-one (54%) patients were on oestrogen treatment as monotherapy at the time of diagnosis. Forty-two patients had a hysterectomy performed within five months, and five patients had a hysterectomy performed 10 to 61 months after diagnosis. A total of 18 out of 57 patients (31.6%) had or developed endometrial carcinoma all with myometrial invasion: 14 stage I with 50% myometrial invasion, and one stage IV. There was no significant difference in age, body mass index, parity or hormone replacement treatment between the group with endometrial carcinoma and the group without endometrial carcinoma. We conclude that unopposed oestrogen treatment and nulliparity are the main risk factors for atypical hyperplasia and that hysterectomy is the appropriate treatment for patients with atypical hyperplasia of the endometrium.