This study was undertaken to clarify the incidence of the vertical transmission of hepatitis C virus (HCV). During the third trimester, 2015 pregnant women were examined as to anti-HCV antibodies. Anti-HCV antibody seropositive women were examined for HCV-RNA in peripheral blood at labor and in breast milk. Their offspring were also examined for HCV-RNA in umbilical cord blood and peripheral blood one week after birth and during subsequent outpatient visits. The following results were obtained: (1) Twelve of the 2015 pregnant women (0.6%) were seropositive for anti-HCV antibodies; (2) Seven of the twelve women (58%) seropositive for anti-HCV antibodies were also seropositive for HCV-RNA; (3) Three newborns of the seven HCV-RNA seropositive women (43%) were found to have HCV-RNA in the cord blood; (4) In the three newborns HCV-RNA had disappeared from the peripheral blood within one month after birth; (5) Two of the seven HCV-RNA seropositive women (29%) had HCV-RNA positive breast milk; (6) The possibility of infection via breast milk was shown in one infant at ten months after birth. Based on these results, it is indicated that HCV vertical transmission is possible in more than half of the HCV-RNA seropositive mothers. However, because of the disappearance of HCV from the infants' peripheral blood, further following study is needed.