Obesity results from a chronic imbalance between energy intake and energy expenditure, with excess calories stored as fat. As such, weight loss has long been considered as a primary goal of treatment for obesity. A surgical treatment of severe obesity such as gastric bypass provides the most dramatic reductions in body weight, and a well-known effect of weight loss is an improvement in insulin sensitivity. However, the molecular mechanism underlying this association remains unclear. Thus, we profiled skeletal muscle of morbidly obese patients before and after gastric bypass surgery. Results from this project will provide global patterns of gene expression with weight loss, which help to understand the pathogenesis of obesity at the molecular level.