Endometrial (uterine) cancer affects over 40,000 women each year in the United States. At this time there are no medications to prevent endometrial cancer. Women who are heavy are at increased risk of this cancer. Progestins have been used to treat endometrial hyperplasia and cancer in women who wanted to retain their fertility or who were not surgical candidates. Oral progestins are associated with side effects such as weight gain, mood changes, headaches, and acne. A device called Mirena has a type of progesterone in it. Because it is placed directly in the uterus it can give a powerful amount of progestin with less side effects. In this project, we will enroll 44 women who are aged 40-50 and who have a body mass index (BMI) greater than 40. (BMI is a measure of obesity. Normal is less than 25.) They will be randomized to either observation or treatment. The women in the treatment group will have a levonorgestrel intrauterine system (Mirena, LNG-IUS) placed for one year. All of the women will have endometrial biopsies and blood work at the beginning and end of the study. All of the women will have an ultrasound at the beginning of the study. The information will then be used to assess whether or not the LNG-IUS will be an effective prevention agent. The women will be contacted once a year for 5 years to see if they have had endometrial hyperplasia or cancer. A special lab study called microarray will be used to see what genes are turned on or off in the uterine lining.