A prospective randomized study was carried out to evaluate doxycycline hyclate in the prevention of hepatic dysfunction following jejunoileal bypass for morbid obesity. Forty-five patients were entered into the study and were observed for a year or more. Patients received either 100 mg of doxycycline hyclate twice daily on postoperative days 5 through 10 and then 100 mg daily for six weeks, or no drug. At six weeks, 12 weeks, and 12 months after the operation, biochemical levels were measured. After one year all patients had a liver biopsy. Doxycycline, in the regimen, was not demonstrated to be of value in the prevention of hepatic dysfunction in these patients. A lack of correlation was demonstrated between the biochemical definition of hepatic dysfunction and liver histopathology.