Thd investigate the genetic background of human T-cell leukemia virus type I (HTLV-I) and II (HTLV-IIhd carriers among South American native Indians, we analyzed HLA DRBI*-DQBI * haplotypes of the virus carriers from Andes highlands and Orinoco lowlands by the PCR-RFLP genotyping method. It was revealed that the HTLV-I-carrying Andes natives had one of the 5 HLA haplotypes: DRBI*-DQBI* 0403-0302, 0802-0402, 0901-0303, 1406-0302 and 0407-0302, and that the Orinoco HTLV-II carriers had one of the 3 HLA haplotypes: DRBI*-DQBI* 1402-0301, 1602-0301 and 0404-0302. The HLA haplotypes of Andes HTLV-I carriers and Orinoco HTLV-II carriers were mutually exclusive. The haplotypes associated with HTLV-I carriers were commonly found among the Andes Indians and Japanese, which is the known HTLV-I endemic population, while the haplotypes associated with HTLV-II carriers were specifically found among the Orinoco Indians and North American Indians, among whom HTLV-II is endemic. These reults suggested that HLA haplotypes might be ethnically segregated among South American natives and might be involved in the susceptibility to HTLV-I and HTLV-II infections.