To assess the rate of mother-to-infant transmission of hepatitis C virus (HCV) and to identify potential risk factors for transmission, we followed up (mean 22.4 months, range 1-7.5 years) a cohort of 291 babies born to anti-HCV-positive mothers, 40 of whom were also HIV coinfected. Seventeen (5.8%) babies acquired HCV infection, but none became icteric. All babies developed chronic HCV infection with 16 babies showing elevated levels of ALT. The rate of transmission was higher in babies born to mothers coinfected with HIV than in those born to mothers with HCV alone (22.5 vs. 3.2%, p < 0.0001). No association was seen between a specific maternal HCV genotype and an increased risk of neonatal infection. The median level of HCV-RNA was higher in mothers who transmitted infection than in those who did not, although the ranges overlapped. In this study, maternal history of chronic liver disease, mode of delivery and type of feeding were not predictive of HCV infection.