Famciclovir (FCV) and lamivudine (LAM) reduce viral replication in patients with recurrent hepatitis B virus (HBV) infection after orthotopic liver transplantation (OLT).Eighteen of 20 patients with insufficient response to FCV were treated with 100 mg LAM daily after OLT. These patients had shown nonresponse (n ؍ 5), partial response (n ؍ 7), or breakthrough (n ؍ 6) during FCV therapy. Despite passive immunoprophylaxis with hepatitis B immunoglobulin after liver transplantation, HBV reinfection had occurred in 14 of 15 transplanted patients. HBV-DNA levels and the regions A to E of the HBV-DNA polymerase gene were analyzed before and after treatment failure to either therapy. Within 4 weeks on LAM, all but 1 patient showed a 95% average reduction of the HBV-DNA level. As with FCV, we did not observe any severe side-effects attributable to LAM. However, 7 patients developed a breakthrough within 12, 29 (n ؍ 2), 32, 37, 54, and 145 weeks under treatment with LAM associated with the methionine-to-valine signature mutation (M552V) in the YMDD motif in all. With FCV, no unique, but a dominant, resistance pattern with the L528M mutation was identified for patients with breakthrough under FCV. In contrast, nonresponders or patients with partial response to FCV did not exhibit such mutations. Our results indicate that the L528M mutation is a risk factor for LAM breakthrough, because breakthrough during LAM occurred earlier in patients with this mutation (50 ؎ 10 weeks vs. 120 ؎ 21 weeks). Because breakthrough on either treatment is frequent for this specific group of patients, the use of combination therapy should be explored. (HEPATOLOGY 1999;30:244-256.)Hepatitis B virus (HBV) infection affects more than 300 million people worldwide. The infection frequently leads to cirrhosis and hepatocellular carcinoma. Orthotopic liver transplantation (OLT) is a therapeutic option for end-stage liver disease. Results of liver transplantation for HBV-related end-stage liver disease are worse compared with other benign indications for liver transplantation, 1 because HBV reinfection often runs a severe course in liver graft recipients, 2,3 with an estimated 3-year survival of only 54%. 4 Treatment with passive immunoprophylaxis with hyperimmunoglobulin against hepatitis B surface antigen (HBIg) has improved the outcome of these patients. 5 In some patients, in particular if pretransplantation replication levels were high, this regime fails to prevent recurrence of HBV. Thus, additional treatment options are needed for these patients .6 Two new oral nucleoside analogues, lamivudine (LAM) and famciclovir (FCV), have been reported as potent inhibitors of hepadnaviral replication in vitro 7,8 and in vivo. 9-12 HBV replicates via an intermediate RNA step through the process of reverse transcription. As a consequence, the mutation rate in the HBV genome is increased compared with other DNA viruses as a result of the lack of proofreading activity in the viral polymerase. Nonresponse or resistance to nucleoside analogues can be rel...