Risk factors affecting vertical hepatitis C virus (HCV) transmission are not completely known, if we exclude maternal HIV coinfection. We hypothesized that immunogenetic factors related to maternal or neonatal HLA profiles may affect HCV vertical transmission. HLA typing (microcytotoxicity assay on blood samples) was performed in 18 infants affected by vertically transmitted HCV infection and in 17 serum-reverted infants. (Serum-reversion is defined as antibody negative by 1 year of age and persistently HCV-RNA negative.) Moreover, HLA typing was performed in 20 mothers. Logistic regression analysis showed a significant negative association between children's HLA-DR13 antigens and risk of HCV vertical transmission (p < 0.01). This association persisted in a model including the maternal HIV status: HLA DR13 and maternal HIV coinfection showed a separate, opposite effect on vertical HCV infection (p < 0.01 and p < 0.001, respectively). The relative risk estimate for the ratio of not-infected to infected children in the presence of DR13 was 8.4 (95% confidence bounds, 1.1-60.8). Breast-feeding did not affect the risk of vertical HCV transmission. Maternal HLA profile did not relate to vertical infection. The present study reveals a significant association between HLA-DR13 and the likelihood of seroreversion in infants born to HCV-infected mothers. The findings of the present study could help in better understanding the pathogenesis of vertical HCV infection and in better identifying the cases at higher risk, which would be useful for the development of prevention strategies. It is possible that DR13 modulates the immune response to viruses, enhancing their clearance and, thus, in the case of HCV, exerting a protective role against the development of vertical infection.