The frequency of radiological sacroiliitis and its association with HLA-B27, Cw1, and Cw2 were determined in Navajo and Hopi Indians coming to a community hospital. In the Navajo, the phenotype frequency of HLA-B27 was 36% and the frequency of sacroiliitis, 11%. In contrast, among the Hopi, the phenotype frequency of HLA-B27 was 9% and the frequency of sacroiliitis, 4%. Of the Navajo men, who had sacroiliitis and were HLA typed, 83% were HLA-B27 positive. The increased frequency of radiological sacroiliitis and were HLA typed, 83% were HLA-B27 positive. The increased frequency of radiological sacroiliitis in the Navajo was felt to be related to the high frequency of HLA-B27 and Reiter's syndrome in this population. HLA-B27, HLA-Cw1, and HLA-Cw2 are known to be in linkage disequilibrium, and about one-half of the men with sacroiliitis from both tribes had concurrence of HLA-B27 and HLA-Cw2.