Extensive clinical data have shown that lamivudine is an effective and safe drug for patients with chronic hepatitis B virus infection. No significant serious side effect has been reported. Four hundred and forty-eight patients with chronic hepatitis B, treated with lamivudine for more than 6 months, were closely monitored. Two patients developed acute myeloid leukaemia during or after lamivudine therapy. The first case developed acute myeloid leukaemia, 1 year after stopping lamivudine therapy, when A529T mutant HBV-DNA was still detectable. The second case achieved complete virological response but suffered from acute myeloid leukaemia during the ninth month of lamivudine treatment. D553N mutant hepatitis B virus was detected in granulocytes of her peripheral blood. Based on our lamivudine therapy data, the calculated incidence of acute myeloid leukaemia in patients during or after lamivudine therapy was higher in males and females than that of the general population. Whether lamivudine-selected viral mutations have enhanced activity/production of transcriptional transactivator and thereby increased the chance of leukaemic transformation of haematopoietic progenitor cells deserves further investigation.