Lamivudine given during the last weeks of pregnancy in women with chronic hepatitis B has been reported to be safe. We report a case of chronic hepatitis B virus (HBV) infection in a newborn, despite suppression of HBV DNA to undetectable levels in the mother by prolonged lamivudine therapy. The newborn had raised alanine aminotransferase concentrations and was positive for HBV DNA at birth which persisted until 9 months of age, despite neonatal vaccination, treatment with hepatitis B immune globulin, and high concentrations of anti-HBs. On HBV DNA sequencing, complete sequence homology and a similar precore mutation was found in the mother and child, indicating vertical transmission. Lamivudine therapy might not prevent perinatal transmission of HBV infection in every newborn.