The rate of transmission of hepatitis C virus (HCV) from patients with chronic hepatitis C to their children was studied. Of the 64 children with a parent with chronic hepatitis C, two (3%) had abnormal alanine aminotransferase (ALT) activities, six (9%) had anti-HCV detected by c100 ELISA, seven (11%) had anti-HCV detected by ELISA-II, and 21 (33%) had HCV-RNA by polymerase chain reaction (PCR). Anti-HCV detected by ELISA-II disappeared within six months in all six infants. Of the five children whose mothers were given a blood transfusion after the child's first birthday, none had anti-HCV or HCV-RNA. In the five families whose elder or eldest offspring had HCV-RNA, all of the younger offspring had HCV-RNA. The vertical transmission rate of HCV was low if judged by the presence of anti-HCV or abnormal ALT values, but the rate was high (33%) if judged by the presence of HCV-RNA.