AIM: To evaluate the sensitivity and specificity of abdominal ultrasound (US) for the diagnosis of hepatic steatosis in severe obese subjects and its relation to histological grade of steatosis. METHODS: A consecutive series of obese patients, who underwent bariatric surgery from October 2004 to May 2005, was selected. Ultrasonography was performed in all patients as part of routine preoperative time and an intraoperative wedge biopsy was obtained at the beginning of the bariatric surgery. The US and histological findings of steatosis were compared, considering histology as the gold standard. RESULTS: The study included 105 patients. The mean age was 37.2+/-10.6 years and 75.2% were female. The histological prevalence of steatosis was 89.5%. The sensitivity and specificity of US in the diagnosis of hepatic steatosis were, respectively, 64.9% (95% CI: 54.9-74.3) and 90.9% (95% CI: 57.1-99.5). The positive predictive value and negative predictive value were, respectively, 98.4% (95% CI: 90.2-99.9) and 23.3% (95% CI: 12.3-39.0). The presence of steatosis on US was associated to advanced grades of steatosis on histology (P=0.016). CONCLUSION: Preoperative abdominal US in our series has not shown to be an accurate method for the diagnosis of hepatic steatosis in severe obese patients. Until another non-invasive method demonstrates better sensitivity and specificity values, histological evaluation may be recommended to these patients undergoing bariatric surgery.