This report describes nucleotide sequence analysis of part of the polymerase gene of hepatitis B virus (HBV) during the development of lamivudine-resistant HBV in five patients who received lamivudine treatment in conjunction with liver transplantation. Samples from patients were analysed before, during and after drug treatment in conjunction with serum HBV quantification by PCR. Lamivudine resistance was found to be associated with L526M and M550V changes in two patients and M550I change in three patients. Other changes associated with lamivudine resistance in some patients were V509I, A546V, S565A and A568T. The effects on HBV surface antigen are also described. Some patients were subsequently treated with famciclovir and/or ganciclovir with variable outcomes. In two out of three patients who stopped lamivudine treatment, reversion (partial or complete) to wild-type virus was observed after about 5 months. In contrast, a complex mixture of mutant viruses emerged in a third patient who stopped lamivudine treatment.