Eighteen cases of Reiter's syndrome were found in a population of approximately 6,000 Navajo Indians. The phenotype frequency of HLA-B27 in this population was found to be 36%. The area in which these people live is endemic for shigellosis, Shigella flexneri being the most common species isolated. Presumably the high frequency of HLA-B27 and shigella infections account for the high incidence and prevalence of Reiter's syndrome in this population.