OBJECTIVE To determine the association between maternal syphilis and HIV mother-to-child transmission (MTCT). METHODS Prospective cohort study. METHODS Pregnant women admitted at Queen Elizabeth Central Hospital (Malawi) in late third trimester were screened for HIV (by HIV rapid tests) and syphilis (by rapid plasma regain test and Treponema pallidum hemagglutination assay). HIV-infected women and their infants received nevirapine, according to the HIVNET 012 protocol. They were followed up at 6 and 12 weeks postpartum. Infant HIV infection was diagnosed by DNA PCR. RESULTS Of the 1155 HIV-infected women enrolled, 1147 had syphilis test results, of whom 92 (8.0%) were infected. Only 751 HIV-positive women delivered live singleton infants who were tested for HIV at birth. Of these, 65 (8.7%) were HIV-infected, suggesting in utero (IU) HIV MTCT. Of the 686 infants who were HIV-negative at birth, 507 were successfully followed up. Of these, 89 (17.6%) became HIV-infected, suggesting intrapartum/postpartum (IP/PP) HIV MTCT. Maternal syphilis was associated with IU HIV MTCT, after adjusting for maternal log10 HIV-1 viral load and low birth weight (LBW) [adjusted relative risk (ARR), 2.77; 95% CI, 1.40-5.46]. Furthermore, maternal syphilis was associated with IP/PP HIV MTCT (ARR, 2.74; 95% CI, 1.58-4.74), after adjusting for recent fever, breast infection, LBW and maternal log10 HIV-1 viral load. CONCLUSIONS Maternal syphilis is associated with IU and IP/PP HIV MTCT. Screening and early treatment of maternal syphilis during pregnancy may reduce pediatric HIV infections.