UNLABELLED: The non-alcoholic steato-hepatitis (NASH) is a common disorder in obese, type 2 diabetics, female and patients with dislipidaemia. Hepatic biochemical test are abnormal. Despite the lack of its own morphological characteristics, NASH can be differentiated from other pathologies, the gold standard for diagnosis is liver biopsy. MATERIAL AND METHODS: We designed a retrolective, comparative, observational and cross-sectional study. Thirty-five obese subjects (11 men and 24 women) who underwent to bariatric surgery and liuer biopsy were included. Data were taken from clinical files, such as anthropometric and biochemical test. Those who had clinical history of related alcohol ingestion or liver damage related to drugs were excluded. A experimented pathologist classified the biopsies according to Brunt classification. Liver slides were classified according to 1) presence of NASH; 2) Inflammation and 3) Fibrosis. Differences between groups were analysed by ANOVA and Spearman correlation. RESULTS: We found differences between women (w) and men (m) for height (m: 1.71 +/- 0.9 vs. w:1.60 +/- 0.09m); weight (m:172.5 +/- 39.1 vs. w:126.9 +/- 24.1kg) and BMI (m:58.2 +/- 9.8 vs. w:49.8 +/- 9) , but not for NASH frequency. Nevertheless subjects with NASH (n = 29, 82.8%o) were older than those without NASH (38.3 +/- 9.6 vs. 29.5 +/- 5.2) and had higher aminotrasferases serum levels (AST: 33.1 +/- 19.2 vs. 23.7 +/- 6.3 ULIL; ALT: 36.5 +/- 19.8 vs. 20.3 +/- 7.6ULIL). NASH pa- tients and those with higher grade of histological inflammation had increment of transaminases and albumin levels. Fibrosis showed correlation only with AST (p = 0.020) and ALT (p = 0.002). CONCLUSION: The NASH frequency in patients who underwent to bariatric surgery for weight reduction is very high (82.8%) and exists correlation among liver test and histological findings but not with clinical because the clinical diagnosis is complicated.