Due to obesity epidemics, nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are becoming the main causes of chronic liver disease in western countries. Nonalcoholic steatohepatitis is a potentially progressive disease that may cause cirrhosis. We analysed liver histology in 505 patients at the time of gastric by-pass surgery. Steatosis was present in 92% of these patients, mild (< 30% of hepatocytes) in 46%, moderate (30-60% of hepatocytes) in 30%, and severe in 23%. Insulin resistance, diabetes, elevated ferritin and elevated liver tests were independent predictors of NASH in the liver these obese patients. Early identification of these factors might help to select patients at risk of NASH in whom liver biopsy should be considered.