Hepatic steatosis, nonalcoholic steatohepatitis and cryptogenetic cirrhosis are frequent in an obese population. Therefore, it is wise to submit all obese patients with significant alterations in hepatic function tests to transparietal liver biopsy. The aim of this study is to determine the hepatic conditions of morbidly obese patients during bariatric surgery by means of a wedge liver biopsy, to avoid any eventual hepatic damage being ascribed to the surgical procedure. This prospective study entails 216 consecutive patients, whose work-up included liver function tests, before undergoing vertical gastroplasty and wedge liver biopsy. Histology was assessed for hepatic steatosis, necroinflammatory activity and liver fibrosis/cirrhosis. Abnormal preoperative liver function tests were detected in 65 patients, in 52 unexpectedly. Histologically, significant steatosis was found in 168 patients (77.8%); necroinflammatory activity in 13 (6.0%); liver fibrosis in 46 (21.3%), 5 of whom (2.3%) were found to have an asymptomatic and unknown liver cirrhosis. In morbidly obese patients, the incidence of histological liver damage is very high, despite acceptable liver function tests. In addition to steatosis, however, a "second hit" to induce necrosis and inflammation, favoring the development of significant fibrosis, is not essential. Being obese is an independent risk factor for liver damage and could contribute to liver fibrosis either alone or in association with other insulting factors. The identification of obese patients with septal fibrosis/cirrhosis, at surgery, is of considerable interest in clinical practice, mainly under the aspect of prognosis and liability.