CCR5, a chemokine receptor, serves as a coreceptor for macrophage-tropic HIV-1 (1-3). A 32-bp deletion within the gene encoding CCR5, CCR5del32, has been shown to prevent HIV-1 infection of T cells in the absence of a wild-type allele. This alteration is present in low frequency in Caucasian populations (4-6). To investigate the effect of CCR5del32 in perinatal HIV-1 transmission and disease progression, two cohorts of perinatally exposed infected and uninfected children were analyzed for the presence of the allele. Polymerase chain reaction (PCR) was used to identify CCR5del32 in prevalent and prospective cases among 144 African American children from New York City and 73 Caucasian children from Barcelona, Spain. HIV-1 transmission; clinical manifestations of disease, including encephalopathy, opportunistic infections, and death before 2 years of age; survival; Centers for Disease Control and Prevention (CDC) classification; and degree of immunosuppression were compared in children with and without CCR5del32. The allele frequency in HIV-1-infected African Americans (0.016) was lower than in Catalan children (0.041). No evidence for a dominant protective effect of CCR5del32 for HIV-1 transmission or disease progression was found in these cohorts.