The contribution of obesity and/or diabetes to liver pathology in the morbidly obese patient is controversial. We studied the liver biopsies of 100 consecutive patients undergoing gastric bypass surgery for morbid obesity. Multiple morphologic parameters were analyzed and graded independently, without knowledge of the clinical history, liver function tests, and oral glucose tolerance results of the patients. Six percent of the entire group demonstrated no fat, 42% mild fat, 20% moderate fat, and 24% severe fatty metamorphosis of the liver. Twenty-three percent of the patients had central vein fibrosis, 23% sinusoidal fibrosis, 19% bridging fibrosis, and 4% cirrhosis. Thirty-six percent of the patients had some degree of steatohepatitis, 66% possessed so-called glycogen nuclei of hepatocytes, 6% had PAS-positive thickening of blood vessels in the portal tracts, and 1% had lipogranulomas. The degree of fatty metamorphosis and fibrosis was analyzed in three separate groups, categorized by the glycemic status of the patient: 46 patients with normal glucose tolerance (NGT), 23 patients with impaired glucose tolerance (IGT), and 31 patients with non-insulin-dependent diabetes mellitus (NIDDM). Increasing severity of fatty metamorphosis from the normoglycemic obese to the diabetic obese patients was found, which was statistically significant by chi 2 analysis. Four of the six patients showing no fatty metamorphosis were normoglycemic. Glycogen nuclei and PAS-positive blood vessels were significantly more prevalent in the diabetic obese than in the normal obese. In conclusion, the distribution of significant liver histopathology in the morbidly obese patient correlates in severity with the degree of impaired glycemic status.