Prophylactic hepatitis B immunoglobulin (HBIg) reduces the risk of reinfection in hepatitis B surface antigen (HBsAg)-positive liver transplant recipients. In the medical center of this study, high-dose HBIg immunoprophylaxis is administered at a fixed dose of 10,000 IU monthly, and in this study, the long-term efficacy of this treatment regimen was examined. Of 52 HBsAg-positive liver transplant recipients, 24 were administered HBIg immunoprophylaxis, and 28 were administered no specific therapy; the 2-year recurrence rates (defined by the reappearance of HBsAg) were 19% and 76%, respectively. Fifty-four percent of the HBIg-treated patients were positive for HBeAg or hepatitis B virus (HBV) DNA (by hybridization assay) pretransplantation. In patients administered monthly HBIg, intrapatient and interpatient variability in trough antibody to HBsAg (anti-HBs) titer was significant, highlighting the potential difficulties of using anti-HBs titer to guide therapy. Trough anti-HBs titers were less in patients who became HBsAg positive than in patients who remained HBsAg-negative (490 vs. 1290 mIU/mL) (P = .0001), reflecting either the cause or effect of HBV reinfection. Of 9 patients who remained HBsAg-negative and who were administered monthly HBIg for at least 1 year, HBV DNA by polymerase chain reaction amplification was detectable in the sera of 67%, the lymphocytes of 50%, and the liver of 57%. In conclusion, a fixed monthly dose of HBIg reduces the recurrence of HBs antigenemia, even in patients with indices of active viral replication pretransplantation. The presence of residual virus in the majority of patients administered HBIg suggests that long-term HBIg administration may be necessary.