Background. Both weight loss and gastrointestinal surgery for obesity can cause liver disease, making their role in the treatment of obesity-related liver disease controversial. Methods. Six hundred eighty-nine severely obese women (n = 55 1) and men (n = 138), BMI = 4 7 9 kg m(-2) (mean +/- SD), without known liver disease, underwent biliopancreatic diversion (BPD) with liver biopsy. Fourteen patients (2%) had cryptogenic cirrhosis, 11 of whom underwent multiple repeat biopsies. After 38 18 kg weight loss, 104 of the 689 patients underwent routine second biopsies during reoperations 41 +/- 25 months after BPD. All biopsy specimens were graded for stealosis, fibrosis, and inflammation by a blinded hepatopathologist. Results. All 689 patients lost weight accompanied by improvements in the metabolic syndrome. Among the 104 patients who underwent reoperation, severe fibrosis (grade 3-5) decreased in 28 whereas mild fibrosis (grade 1-2) appeared in 42. Increased fibrosis was related to low-normal serum albumin, uncontrolled diarrhea, low intake of alcohol, and menopausal status. Fibrosis and inflammation decreased over time (P <.01). The 11 patients with cirrhosis exhibited decreased fibrosis from a mean grade 5 to grade 3, as well as reduced inflammation, Mallory bodies, and glycogenated nuclei. Seven patients had disappearance and 2 regression of nodules and fibrous bridging. Conclusions. The metabolic syndrome of obesity is a determinant of liver fibrosis and cirrhosis, treatable by substantial weight loss after malabsorptive surgery.