Patients with chronic hepatitis B virus (HBV) infection have a defective HBV-specific immune response, and the spontaneous development of antibody against hepatitis B surface antigen (anti-HBs) after liver transplantation has not been observed. We report the spontaneous production of anti-HBs in 21 of 50 (42%) patients receiving lamivudine monoprophylaxis after liver transplantation. Seroconversion to anti-HBs status (>10 mIU/mL) was found at a median of 8 days (range, 1 to 43 days) after transplantation. In each case, serial serum samples showed a >100% increase in antibody titer as compared with that of day 7 after transplantation in the absence of any blood product transfusion. The anti-HBs titer increased to a maximum within 3 months, and the peak titer was <100 mIU/mL in 10 patients, 100 to 1000 mIU/mL in 5 patients, and >1,000 mIU/mL in 6 patients. In 12 patients, anti-HBs disappeared from serum at a median of 201 days (range, 24 to 414 days), whereas the other 9 patients remained positive for anti-HBs at a median of 221 days (range, 94 to 1,025 days) after transplantation. Patients in whom anti-HBs in serum developed had a more rapid clearance of serum hepatitis B surface antigen (HBsAg) (log rank test, P ؍ .011). Using logistic regression analysis, the only predictor of anti-HBs production was an HBV-immune donor (odds ratio, 18.9; 95% confidence interval, 3.2 to 112.4; P ؍ .001). In conclusion, patients who undergo liver transplantation for chronic hepatitis B using lamivudine prophylaxis may develop anti-HBs spontaneously. The antibody is likely to be of donor origin, suggesting the possibility of adoptive immunity transfer through a liver graft. L iver transplantation in patients who are chronic carriers of hepatitis B surface antigen (HBsAg) is associated with a high risk of graft reinfection that rapidly progresses to graft failure. 1 Polyclonal hepatitis B immunoglobulin (HBIg) is an effective form of prophylaxis that has been widely adopted. [2][3][4] More recently, the advent of nucleoside analog reverse transcriptase inhibitor, such as lamivudine, which inhibits hepatitis B virus (HBV) replication, has provided an alternative approach. 5 As a result of a significant rate of failure with single-agent prophylaxis with either HBIg or lamivudine, 2,6,7 combination therapy using both HBIg and lamivudine is increasingly regarded as the most effective approach. [8][9][10][11] Nevertheless, both forms of prophylaxis, either alone or in combination, do not completely eradicate the virus, but keep the viral load at a level low enough to prevent clinical disease. 5,7,12 Therefore, there is potentially an indefinite risk of graft infection on discontinuation of the prophylaxis unless the host can resume an adequate immune response.The development of antibody against hepatitis B surface antigen (anti-HBs) has been shown to be important in protecting against HBV infection. In patients with acute HBV infection, seroconversion to anti-HBs status is associated with complete recovery and lifelong immunity. 13...