BACKGROUND The role of serum hepatitis C virus (HCV) load in infectivity, disease activity, and response to interferon treatment has been investigated in adults, and controversial results have been obtained. Little is known about HCV load in infants and children with HCV infection. PURPOSE To investigate the relation between HCV load in serum and features of associated liver disease in infants and children with HCV infection. METHODS Hepatitis C viral load was investigated in serial samples in 43 children with chronic HCV infection, including 32 patients aged 4 to 16 years infected by different routes and 11 vertically infected infants observed prospectively since birth. RESULTS Overall viremia ranged between 2.7 and 6.9 log copies/ml (median, 5.56 log/ml) and fluctuated slightly during the follow-up. Median HCV RNA levels did not significantly differ among infants, children, and adolescents. Viral load was also independent of sex, route of infection, clinical manifestation, alanine aminotransferase levels, and liver histology. All 11 perinatally infected children became chronic HCV carriers, whatever their initial viral load; retrospective testing of sera taken in the first day of life in three infants showed high viremia levels. CONCLUSIONS Viremia levels observed in children were similar to those reported in adults, were independent of age, biochemical activity of liver disease, and chronicity of infection. They were also relatively stable, suggesting that serial measurement of viral load is useless in untreated infants and children. The detection of viremia at birth in children in whom chronic hepatitis developed later suggests the possibility of in utero infection.