Background &Aims: In severely obese patients, factors implicated in the evolution of severe steatosis after bariatric surgery remain unresolved. Our aim was to determine whether insulin resistance (IR) influences the histologic effects induced by bariatric surgery. Methods: We prospectively included 185 severely obese patients (body mass index ≥35 kg/m2) referred for bariatric surgery. The evolution of IR (IR index = 1/quantitative insulin sensitivity check index) and liver injury with consecutive biopsy was concomitantly assessed before and 1 year after surgery. Results: At preoperative biopsy, 27%of severely obese patients disclosed severe steatosis (≥60%). The alanine aminotransferase (P = .01) and IR indexes (P = .04)were independent predictive factors of severe steatosis at baseline. One year after surgery, surgical treatment induced a decrease in body mass index (9.5 kg/m2; P < .0001), steatosis score (8.5%; P < .0001), and IR index (0.29; P < .0001). The preoperative IR index (P = .01) and preoperative steatosis (P = .006) were independent predictive factors in the persistence of severe steatosis after surgery. Moderate or severe steatosis was more frequently observed in patients who had conserved a higher IR index after surgery than in patients who had improved their IR index (44%vs 20.2%; P = .04). Conclusions:: IR was independently associated with severe steatosis and predicted its persistence after surgery. The amelioration of IR after surgery is associated with a decrease in the amount of fat. Taken together, the results of this prospective study in severely obese patients demonstrate that severe steatosis and its evolution after surgery are intimately connected with IR. Background & Aims: In severely obese patients, factors implicated in the evolution of severe steatosis after bariatric surgery remain unresolved. Our aim was to determine whether insulin resistance (IR) influences the histologic effects induced by bariatric surgery. Methods: We prospectively included 185 severely obese The evolution of IR (IR index = 1 / quantitative insulin sensitivity check index) and liver injury with consecutive biopsy was concomitantly assessed before and 1 year after surgery. Results: At a prioroperative biopsy, 27% of severely obese patients were reported severe steatosis (≥60%). The alanine aminotransferase (P = .01) and IR indexes (P = .04) were independent predictive factors of severe steatosis at baseline. One year after surgery, surgical treatment induced a decrease in body mass index (9.5 kg / m2; P <.0001), steatosis score (8.5%; P <.0001), and IR index inde Moderate or severe steatosis was more frequently observed in patients who had conserved a higher IR index after surgery than in (P = .01) and preoperative steatosis (P = .006) were independent predictive factors in the persistence of severe steatosis after surgery Patients who had improved their IR index (44% vs 20.2%; P = .04). Conclusions :: IR was independently associated with severe steatosis and predicted its persistence after surgery. The amelioration of IR after surgery is associated with a decrease in the amount of fat. Taken together, the results of this prospective study in severely obese patients demonstrate that severe steatosis and its evolution after surgery are intimately connected with IR.