Obesity is a significant risk factor for the development of endometrial hyperplasia and cancer. More conservative prevention and management strategies are attractive due to the increased surgical risk and complication rates associated with obesity. The Levonorgestrel-releasing intrauterine system (LNG-IUS, Mirena) has been shown to reduce the risk of developing endometrial cancer. The recent joint Green Top Guideline on the Management of Endometrial Hyperplasia published by the Royal College of Obstetricians and Gynaecologists (RCOG) with the British Society for Gynaecological Endoscopy (BSGE) recommends the LNG-IUS for the medical management of endometrial hyperplasia without atypia. This case study reports on the development of endometrioid adenocarcinoma despite the presence of an LNG-IUS following a negative hysteroscopy in a 56-year-old woman with morbid obesity. This report highlights the need for patients and clinicians to remain vigilant to the early warning signs of developing endometrial cancer, especially in those at an increased risk secondary to obesity.