In women with hereditary non polyposis colorectal carcinoma (HNPCC) an annual gynaecological surveillance has been recommended because of an increased lifetime risk of developing endometrial and ovarian carcinoma. The aim of this study was to assess the efficacy of gynaecological surveillance with regard to endometrial and ovarian carcinoma. Included were women from families that fulfilled the revised Amsterdam criteria for HNPCC or who showed a proven mutation in one of the mismatch repair genes. An annual gynaecological surveillance was performed (transvaginal ultrasound (TVU) and CA 125 assessment). From January 2006 on, routine endometrial sampling was included. In a total number of 100 women 285 surveillance visits were performed. Among these, in 64 visits routine endometrial samplings were performed: three atypical hyperplasias and one endometrial carcinoma were diagnosed. This was significantly more than the atypical hyperplasia and two endometrial carcinomas that were detected after 28 samples performed because of abnormal surveillance results in 221 visits. There were no interval carcinomas. One invasive ovarian carcinoma stage IIIC was diagnosed at ovarian surveillance. Endometrial surveillance with routine endometrial sampling in women with HNPCC is more efficient in diagnosing endometrial (pre)malignancies than TVU only. Ovarian surveillance is not capable of diagnosing early stage ovarian carcinoma. Prophylactic hysterectomy in HNPCC should be restricted to women in whom abdominal surgery for other reasons is performed and to those with particularly increased risk such as MSH6 mutation carriers and/or women with multiple relatives with endometrial carcinoma.