Obesity is a frequent disease with an increased prevalence, estimated to 12.4% in France (OBEPI study 2006). Non alcoholic fatty liver disease (NAFLD) and non alcoholic steatohepatitis (NASH) both complicate morbid obesity and could be responsible for the development of liver fibrosis and consequently of liver cirrhosis. Bariatric surgery has been widely used in obese patients to reduce their weight but also to decrease morbidity conditions related to obesity. It could be involved in an improvement in liver lesions observed in obese patients. To evaluate the severity of liver lesions, the gold standard is pathological examination of the liver using percutaneous or surgical biopsies, with a scoring called Fibrosis score stage. Alternative less invasive techniques have been developed to evaluate liver lesions. These tests are commonly used in clinical hepatology and consisted of serum levels determination of fibrosis markers (Fibrotest, Transthyretin).