Endometriosis affects a 10 % of women during their reproductive years. Unequoral statistics concerning the incidence of adenomyosis are not available although a combined occurrence of both diseases is found in a 20 % of cases. The risk that malignancy arises from endometrioid tissue typical for endometriosis is between a 0.3-1 %. 75 % of these malignancies are ovarian cancer in conjunction with pre-existing ovarian endometriosis; less frequently extraovarian malignancies are found. The development of malignancy of adenomyosis is very rarely reported. In this report we present the case of a 35 year old patient who suffered from both, endometriosis and adenomyosis and who underwent a therapy using GnRH analogues. After five months and before the completion of the therapy a hysterectomy with conservation of the ovaries was performed at the request of the patient (carcinophobia). The histology confirmed the diagnosis of adenomyosis and demonstrated the unexpected finding of an endometrium carcinoma. This latter arose from a complex atypical hyperplasia surrounded by hypoplastic endometrium. There is some evidence that suggests a slightly elevated risk of breast and ovarian cancer as well as haematological malignancies amongst patients with endometriosis. However, there does not appear to be an increased risk of endometrial carcinoma. Adipositas leads to an increased risk for the development of endometrial carcinoma due to the increased conversion of testosterone to estrone in fat. The peripheral synthesis of estrone is unaffected by GnRHa-therapy. A progesterone containing HRT should be added to a GnRHa-therapy in overweight patients to prevent the development of endometrial hyperplasia and/or carcinoma. In conclusion a careful indication has to be made for GnRHa-therapy in overweight patients and before and during the therapy high resolution ultrasound scan should be performed to evaluate the endometrium in those patients.