In this study, we sought to determine whether or not elevated levels of ethanol were present in the systemic circulation, resulting from endogenous ethanol production, which might contribute to the hepatic damage following jejunoileal bypass for morbid obesity. Venous serum samples for assay of ethanol by gas-liquid chromatography were obtained in 8 normal subjects, 9 obese patients prior to surgery, 20 obese patients 2 weeks to 40 months after jejunoileal bypass, and in 2 dogs before and after jejunoileal bypass. Ethanol was detected after jejunoileal bypass in only 7 of the 20 patients and in the 2 dogs. Serum ethanol concentrations ranged from 0.15 to 4.12 mg/100 ml with a mean of 1.18 +/- 1.59 (SD)( mg/100 ml in the 7 patients and ranged from 0.20 to 2.23 mg/100 ml in the dogs. Incubation of the contents of the bypassed intestine of a dog with dextrose resulted in the production of significant amounts of ethanol. However, there was no correlation between the presence of ethanol in the serum and liver histology, when liver biopsy was obtained, postoperatively. Since ethanol was detected in the serum in only small concentrations and in only one-third of the patients, it is unlikely that ethanol production by bacteria in the intestine is of significance in the pathogenesis of liver disease following jejunoileal bypass.