BACKGROUND The authors analyze and describe the hepatic histopathology of patients with morbid obesity submitted to gastric bypass. METHODS A liver biopsy was performed in 112 patients at the beginning of gastric bypass, and clinical data (gender, age, BMI and associated diseases) were obtained from the patient records. RESULTS Among the patients, 74.1% (n=83) were females and 25.9% (n=29) were males, age 39.59+/-10.12 years, with BMI 48.79+/-7.54. Regarding the diseases associated with morbid obesity, arterial hypertension was observed in 67.9% (n=76) of the patients, joint diseases in 55.4% (n=62), dyslipidemia in 40.2% (n=45), and diabetes type 2 in 27.7% (n=31). 111 patients (99.1%) showed different types of non-alcoholic fatty liver disease (NAFLD) (types 1, 2, 3 and 4), with types 3 and 4 accounting for 57.7% (n=64). A significant association was observed between NAFLD type 4 and diabetes type 2 (P=0.018). The presence of glycogen nuclei was significantly correlated with steatosis (P=0.000), lobular inflammation (P=0.019) and perisinusoidal fibrosis (P=0.000). CONCLUSIONS 1) Liver biopsy taken at gastric bypass reveals different types of NAFLD; 2) Diabetes type 2 is associated with NAFLD type 4; 3) Glycogen nuclei are correlated with steatosis, lobular inflammation and perisinusoidal fibrosis; 4) Steatosis represents the basic injury in NAFLD and is associated with hepato-cellular ballooning. 5) In view of the frequency of hepatic alterations, a biopsy should be part of the surgical protocol of morbid obesity, this method being the only one able to determine the presence of lesions and to stage fibrosis.