Hepatitis B virus (HBV) DNA is detectable in a number of liver transplant candidates who are negative for hepatitis B surface antigen (HBsAg). After liver transplantation (LT), such patients may have molecular and/or serologic evidence of HBV replication. However, clinical disease from reactivation of occult HBV infection after LT has not been described. We report a patient who underwent LT for cryptogenic cirrhosis and had to be retransplanted twice for hepatic artery thrombosis. The patient was negative for HBsAg and positive for anti-hepatitis B core (HBc) and anti-HBs before all LT procedures and developed acute hepatitis B shortly after receiving the third graft. The HBV strain isolated at that time exhibited an unusual in frame insertion of a CAG motif within the HBV polymerase (HBV INSϩ ). HBV INSϩ was detected retrospectively as a minor species in pretransplantation sera and the explanted native liver by insertion-specific polymerase chain reaction. This case in an occult HBV carrier shows that clinically apparent, endogenous reinfection of the graft may occur with minor HBV variants that are not detectable in pretransplantation samples by standard diagnostic procedures. This has implications for the analysis of sources of acute hepatitis B in patients after LT Hepatitis B after liver transplantation (LT) greatly affects patient outcome. In principle, 2 modes of infection are possible: (1) endogenous reinfection in patients who were transplanted for hepatitis B, and (2) exogenous or de novo infection, mostly as a result of nosocomial transmission of hepatitis B virus (HBV) through the liver allograft, blood products, and other exogenous sources. Although the prevalence of de novo infection with HBV is only 3.5% 1 and has a good prognosis in most cases, endogenous reinfection is frequent and may lead to severe hepatitis. 2 A prerequisite for endogenous reinfection after LT is the presence of active hepatitis B before transplantation. The incidence of reinfection after LT depends on the level of pretransplant viremia and the prophylactic measures taken. [3][4][5][6] Few cases of reinfection have been described in patients with serologic signs of previous hepatitis B. [7][8][9] These patients were negative for hepatitis B surface antigen (HBsAg) and positive for anti-hepatitis B core antigen (HBc), but negative for anti-HBs. In recipients who are positive for both anti-HBs and anti-HBc antibodies before LT, to our knowledge, clinically apparent reinfection has not been described yet. By definition, these patients have resolved hepatitis B and can be divided into 2 clinically important groups in the transplantation setting. First, the vast majority of these individuals are negative for serum HBV DNA by polymerase chain reaction (PCR). In the second group, serum HBV DNA can be detected by PCR and these patients are referred to as occult HBV carriers. The outcome of LT in such carriers has yet been investigated in only few studies. None of the study patients experienced clinically apparent reinfection with H...