Background: Overweight and obesity are the most significant risk factors for the development of hepatic steatosis, non-alcoholic steatohepatitis (NASH) and non-alcoholic fatty liver disease (NAFLD) in children and adults. Both have been increasingly implicated in the genesis of hepatic fibrosis and cirrhosis. However, no consensus exists about whether weight reduction may reverse this process. Methods: To assess the effect of obesity surgery on the histological evolution of NASH, diagnosed in 284 morbidly obese patients by routine liver biopsy (‘first’ biopsy) performed during bariatric surgery, we performed a ‘second’ biopsy after 18.6 � 8.3 months in 116 patients (109 female, 7 male). 68 patients underwent Roux-en-Y gastric bypass, 38 adjustable gastric banding and 16 biliopancreatic diversion with duodenal switch (BPD-DS). The second biopsy was taken during CHE (102) and relaparoscopy for suspected complications and revisional surgeries (12). All primary and secondary surgical procedures were done laparoscopically without intraoperative or postoperative major complications. All comorbidities were recorded pre- und postoperatively. Results: From the first to the second biopsy, BMI decreased from 55.2 � 8.3 to 30.5 � 6.6 kg/m², arterial hypertension decreased from 65 to 43%, and type 2 diabetes decreased from 42 to 2%. On the first biopsy, non-alcoholic fatty liver disease (NAFLD) type 3 was observed in 186 patients (65.5%) and type 4 in 82 (28.9%). The second biopsy revealed complete regression of NAFLD in 89 patients (82.8%) and only 16 (13.8%) still had NAFLD type 1 (mild steatosis without inflammation). These two patients with NAFLD type 3 had adjustable gastric banding with insufficient weight loss (EWL <50%) in history. Complete regression of necroinflammatory activity was observed in 108 patients (93.1%). Among the 12 patients presenting fibrosis in the first biopsy, complete remission was observed in 10 and improvement in 2. Two patients with ascites during BPD-DS showed complete remission within 9–15 months. Two continued to show the same degree of fibrosis without evidence of disease activity. No worsening of steatosis, necroinflammatory activity or fibrosis was observed in any of the patients, and none progressed to cirrhosis. Conclusion: Obesity surgery improves steatosis, necroinflammatory activity and hepatic fibrosis in patients with morbid obesity and NASH. The improvement of all obesity-related comorbidities can be confirmed.