The aim of this study is to evaluate the evolution of fibrosis scores during weight loss in a population of patients operated on for morbid obesity ( BMI > 40kg/m2 or BMI> 35kg/m2 with comorbidity). The primary end point is to evaluate the concordance between Fibrotest and transthyretin serum levels. The number of patients has been calculated on the basis of a concordance ( Kendall test) superior to 70% in patients with grade F1 fibrosis that represents about 40% of morbid obese patients. A total of 255 patients has to be enrolled in this study. According to the number of patients operated on in the 3 centres (>150/year), the time for recruitment is 12 months. Secondary end points will evaluate in these patients the excess weight loss, the improvement in fibrosis liver tests and the correlation with improvement in other obesity- related conditions, according to surgical procedures.