The aims of this pilot study were to evaluate the safety and efficacy of interferon-α2b for treatment of hepatitis C virus infection in liver transplant recipients, to monitor changes in hepatitis C virus RNA levels with treatment and to determine pretreatment parameters predictive of a complete response. Eighteen patients with documented hepatitis C virus viremia and histological evidence of hepatitis after liver transplantation received 3 million units of α interferon three times weekly for at least 4 mo. Pretreatment serum aminotransferase levels were at least 1.5 times the upper limit of normal and no patient had concomitant hepatitis B virus infection. Response to therapy was defined as normalization of both aspartate and alanine aminotransferase at the end of treatment. Five patients (28%) had a complete response, whereas 13 (72%) had persistent elevation of one or both aminotransferases (nonresponders). At the end of therapy, hepatitis C virus RNA levels were reduced in both responders and nonresponders (p = 0.043 and 0.039, respectively by Wilcoxon signed rank test). After cessation of treatment, aminotransferases remained normal in four of five responders but serum hepatitis C virus RNA levels returned to pretreatment levels in responders and nonresponders. There was no significant change in histological score with therapy. Responders were more likely than nonresponders to have low pretreatment hepatitis C virus RNA levels and low serum bilirubin (p = 0.0004 and 0.0077, respectively). Responders tended to have a prolonged interval between transplantation and initiation of therapy (p = 0.10 by rank logistic regression analysis). Side effects resulted in early cessation of therapy in two patients and dose reduction in six. Histological evidence of possible rejection was seen in one patient. We conclude that normalization of liver enzymes can be seen in patients with posttransplant hepatitis C virus infection on interferon therapy and that treatment is associated with fall in hepatitis C virus RNA levels, regardless of biochemical response. However, virological changes are transient. Pretreatment variables such as hepatitis C virus RNA levels and serum bilirubin may aid in the selection of patients more likely to respond to therapy and the design of future controlled clinical trials. (Hepatology 1994;20:773–779).