Atypical complex hyperplasia (ACH) is an increase of the endometrial thickness with a risk of progression to endometrial carcinoma. The risk of progression of the ACH to endometrial carcinoma varies from 29-100%, depending on the author. It is very important to make a differential diagnosis between ACH and well differentiated endometrial carcinoma, as following biopsy or curettage an incidence of 15-43% endometrial carcinoma has been found in the hysterectomy specimen. Objectives To analyze the coexistence of ACH and endometrial adenocarcinoma in the hysterectomy specimen, and to assess current practise when faced with ACH. Material and method A retrospective study of 15 patients with ACH, from May 2001 until March 2004. Endometrial biopsy was taken during hysteroscopy, and they had hysterectomy with double adnexectomy and peritoneal washing. Results Eight cases (53%) maintained the preoperative diagnosis of ACH. The other 7 hysterectomy specimens had well differentiated endometrial adenocarcinoma, FIGO grade I. Of these 7 cases of adenocarcinoma (20% of the total operations), 3 were confirmed as having myometrial invasion in less than 50% of its thickness (FIGO grade Ib) No invasion was found in the other 4 adenocarcinomas (FIGO grade Ia). Conclusions These results should highlight the need for strategies to resolve the cases of under staging, with an extension study following diagnosis of ACH, similar to that used in endometrial carcinoma to avoid incomplete treatments. Intra-operative biopsy of the hysterectomy specimen would allow pelvic lymph node dissection to be carried out at the same time if myometrial invasion was confirmed.