The purpose of this study was to evaluate caffeine clearance, a quantitative measurement of metabolizing capacity of the liver, in chronic hepatitis B and C before and after interferon treatment. Biochemical test using AST and ALT, virological test, and caffeine clearance were measured in eighteen patients with chronic hepatitis B and five patients with chronic hepatitis C at pre and post treatment with interferon. Caffeine clearance was determined in each patient using two point analysis following a 3.5 mg/kg oral administration of caffeine solution. Blood samples were subsequently collected at 12 and 16 hours after caffeine administration and assayed for serum caffeine level by HPLC technique. Clearance was calculated using the equation of Cl = Kel x Vd. It was found that caffeine clearances determined before and after interferon treatment were not significantly different in both chronic hepatitis B and C inspite of biochemical and virological responses after therapy. Caffeine clearance change in two diffferent groups of chronic hepatitis B defined as biochemical response and nonresponse were compared. Although caffeine clearance change between responders and nonresponders to interferon treatment was not significantly different, it tended to increase in those patients who had biochemical response to interferon. It appeared that metabolic capacity of the liver does not change with interferon therapy inspite of biochemical and virological responses.