BACKGROUND: Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disorder in industrialized countries. Nonalcoholic steatohepatitis is the fastest growing cause for liver failure. Bariatric surgery represents a treatment option for NAFLD with an established effect on liver histology. OBJECTIVES: We aimed to assess the impact of bariatric surgery on standardized liver histology using the NAFLD activity score. SETTING: Retrospective comparison of metabolic data before and after bariatric surgery and comparison of sleeve gastrectomy and Roux-en-Y gastric bypass. The study was performed in an academic center, the university hospital Schleswig-Holstein in Kiel, Germany. METHODS: Between 2009 and 2012, bariatric surgery was performed in 257 patients according to the national guidelines, and a liver biopsy was obtained in 150 of these patients during surgery. A follow-up biopsy was available in 53 of these patients at a median of 192 days. Liver histology was analyzed using the NAFLD activity score. In this subgroup of 53 patients an analysis of the metabolic improvement after bariatric surgery and a comparative analysis between the 2 different operative procedures was performed. RESULTS: The study cohort showed improvement of preoperative pathologic liver histology findings after operative procedures took place. Both surgery methods improved the NAFLD activity score significantly, all improvement -2.0 (confidence interval -2.5 to -1.0; P < .001); Roux-en-Y gastric bypass, improvement -1.0 (confidence interval -2.0 to -.0; P=.038); sleeve gastrectomy, improvement -2.5 (confidence interval -3.5 to -1.5; P < .001). No differences were found with regard to histologic recovery between gastric bypass and sleeve gastrectomy (P = .22). CONCLUSIONS: Bariatric surgery significantly improves NAFLD.