To determine if the 32-bp deletion of the chemokine receptor CCR5 (delta32ccr5) protects against mother-to-infant transmission of HIV-1, specimens from all uninfected and infected children who were perinatally exposed to HIV-1 and observed since 1988 and whose mothers did not take zidovudine were assessed for delta32ccr5. The CCR5 genotype was determined using polymerase chain reaction (PCR) for 122 subjects, of whom 73 were HIV-1 infected and 49 were perinatally exposed but uninfected; 70% and 71%, respectively, were Caucasian. Eleven of 73 (15%) infected children and 4 of 49 (8%) exposed uninfected children were CCR5/delta32ccr5 heterozygotes (p = 0.40). Among subjects who had at least one Caucasian parent or grandparent, 11 of 51 (22%) HIV-1-infected persons and 4 of 35 (11%) uninfected persons were heterozygotes. None were homozygous for the delta32ccr5 allele. The estimated relative risk for mother-to-infant HIV-1 transmission in heterozygotes was 2.0. Furthermore, the 95% confidence interval (0.6, 7.3) suggested that it is unlikely that the true relative risk was <0.6. Thus, the infant CCR5/delta32ccr5 heterozygous genotype was not associated with a diminished risk of perinatally acquired HIV-1 infection.