A 14-year-old girl with chronic hepatitis B had seroconversion from hepatitis B e antigen to antibody and achieved biochemical remission after 2 years. The disease reactivated 9 years later when a precore mutant had become the prevalent hepatitis B virus strain in serum. These results suggest that selection of a precore mutant might induce reactivation during adult life of chronic hepatitis B acquired in childhood, thus worsening the prognosis.