HLA haplotypes of 27 patients with human T-lymphotropic virus type I (HTLV-I)-associated myelopathy (HAM) and 12 patients with adult T-cell leukemia/lymphoma (ATLL) were examined by analyzing HLA types of the patients and their family members. Either A11Bw54Cw1DR4DQw3, A24Bw54Cw1DR4DQ-, A24B7Cw7DR1DQw1, or A24Bw52Cw-DR2DQw1 and the related haplotypes were found in 70% of cases with HAM. None of these "HAM-associated" haplotypes was found in patients with ATLL. HLA haplotypes made up of HLA components of A26Bw62Cw3DR5DQw3 and one particular haplotype of Aw33B44Cw-DRw6DQw1 were associated with the ATLL haplotypes. These "ATLL-associated" haplotypes were also found in the patients with HAM who had no previous history of blood transfusion. The in vitro cultures of peripheral blood lymphocytes with HTLV-I virion antigens revealed that the response with HAM peripheral blood lymphocytes was remarkably higher than that with ATLL peripheral blood lymphocytes. Based on this HTLV-I-specific immune responsiveness, we can segregate the high responders in HAM (14 of 16 cases) and the low responders in ATLL (6 of 7 cases). The existence of high and low responders was also confirmed by the normal healthy individuals, whose responses were segregated with HAM-associated and ATLL-associated haplotypes. These results suggested that two ethnic groups in southern Kyushu may get the two different diseases, HAM and ATLL, because of their different immunogenetic backgrounds. The high immune response to HTLV-I seems to be an important genetic factor in the development of HAM.