A possible correlation between the rate of vertical transmission of HIV-1 and the presence of the defective HIV co-receptor gene delta 32ccr5 in the chromosomes of infants born to HIV-positive mothers was assessed. The prevalence and genotypic distribution of the delta 32ccr5 gene were studied in 451 uninfected and 225 HIV-1-infected adults and 79 children born to HIV-1-positive mothers in Austria (45 uninfected and 34 infected by vertical transmission). As expected in a Caucasian population, the delta 32ccr5 allele was found in uninfected Austrians at a frequency of 10% (17.3% heterozygotes and 1.3% delta 32ccr5/ delta 32ccr5 homozygotes, consistent with the expected Hardy-Weinberg distribution). The mutant allele frequency was 11.1% in uninfected children (17.8% heterozygotes, 2.2% homozygotes) and 9.6% in HIV-positive adults (19.1% heterozygotes but no delta 32ccr5/delta 32ccr5 homozygotes). Among the group of 34 vertically infected children, however, there were only two heterozygotes and no delta 32ccr5/delta 32ccr5 homozygotes, corresponding to a significantly reduced mutant allele frequency of 2.9% (P = 0.05 compared to HIV-negative children). These results suggest that CCR5/delta 32ccr5 heterozygous children are less susceptible to vertical transmission of HIV-1. The data also support the hypothesis that delta 32ccr5 homozygous individuals are resistant to HIV-1 infection.