We report the case of an in-situ endometrial adenocarcinoma with severe atypical hyperplasia which developed while the patient was on tamoxifen only 1 year after endometrial resection for benign pathology. A 55-year-old woman received tamoxifen as adjuvant therapy for breast cancer treated in 1994. At that time, a benign endometrial polyp was also removed before initiating tamoxifen treatment. In 1997, endometrial resection was performed for benign pathology (atrophic cystic endometrium). In 1998, an endometrial adenocarcinoma was diagnosed. This case illustrates that endometrial resection does not protect women taking tamoxifen against subsequent development of severe atypical lesions.