PURPOSE: To determine the prevalence of non alcoholic fatty liver disease (NAFLD) and non alcoholic steatohepatitis (NASH) in morbidly obese patients undergoing bariatric surgery and to identify risk factors associated with the disease spectrum. METHODS: Liver biopsy was performed in 60 patients who underwent bariatric surgery, after other causes of liver disease were excluded. Clinical, biochemical and histological features were evaluated. RESULTS: NAFLD was detected in fifty-seven patients (95%) of the sample and forty patients (66.7%) of the total sample met the criteria for NASH. Perisinusoidal fibrosis was only found in three (7.5%) patients with NASH. The γGT was an independent predictive factor associated with the degree of hepatic steatosis. The variables such as dyslipidemia and ALT were independently associated with the presence of Mallory's corpuscles with the following values, respectively, OR 0, 05, 95% CI 0.002 to 0.75, P = 0.031 and OR 10, 99, 95% CI 1.44 to 83.93, P = 0.021. CONCLUSIONS: Non alcoholic fatty liver disease seems to be an obese-related condition with approximately half of asymptomatic morbidly obese patients having histological NASH. The γGT was an independent predictor of the degree of steatosis.