OBJECTIVE The aim of this study was to clarify the aetiology of apparent de novo HBV infection after liver transplantation. METHODS Twenty out of 570 HBsAg negative patients (3.5%) became HBsAg positive after transplantation and were studied. Donor and recipient sera were retrospectively tested for HBsAg, anti-HBs, anti-HBc, and HBV DNA by PCR. Donor and recipient livers were tested for HBV DNA by PCR on paraffin-embedded tissue. RESULTS Group 1: HBV infection of donor origin (eight patients): one donor serum was HBsAg positive, three were serum HBV DNA positive, four were liver HBV DNA positive. Group 2: reactivation of latent HBV infection (eight patients) with detection of HBV DNA in pretransplant serum (seven patients) or in native liver (one patient): three were anti-HBs positive, two anti-HBc positive, and three with fulminant hepatitis had no serological HBV markers. Group 3: undetermined origin (four patients) defined by absence of HBV DNA in pretransplant donor and/or recipient sera and liver; however, acquired infection was suspected from two anti-HBs and anti-HBc positive donors. Two patients became HBsAg negative, and five HBV DNA negative. One died from HBV-cirrhosis and two were retransplanted. In the others, the last histology showed cirrhosis (three), chronic hepatitis (nine), acute hepatitis (one), and non-specific change (four patients). CONCLUSIONS The prevalence of de novo HBV infection in liver transplant patients was 3.5%; the aetiology was determined in 16/20 patients: from the donor in eight, and from the recipient in eight. One should be cautious when donors or recipients are anti-HBc or both anti-HBs and anti-HBc positive.