The association of class I and II HLA antigens with rheumatic fever and its manifestations was examined in 72 patients, including 48 blacks and 24 Caucasians. No significant association was found between class I antigens and rheumatic fever. In contrast, HLA-DR2 and HLA-DR4 phenotypes were encountered in a significantly higher frequency in black and Caucasian patients with rheumatic fever, respectively, compared with the control populations (P less than 0.005). The most significant association (P less than 0.005) of these DR antigens with a major manifestation of rheumatic fever was found for mitral insufficiency. In addition, a significant association was encountered between persistent elevation of antibody to the group A streptococcal carbohydrate and HLA-DR4 in Caucasian patients (P less than 0.04) or HLA-DR2 in the black patients (P less than 0.001). The frequency of HLA-DR2/4 heterozygotes among patients with rheumatic fever did not differ significantly from controls. These findings support the concept of a genetically determined susceptibility to rheumatic fever and, particularly, to rheumatic heart disease. The association of the clinical manifestations of rheumatic fever and the immune hyperresponsiveness to a streptococcal antigen could be ascribed to a disease-associated immune-response gene which is in linkage disequilibrium with the DR2 and DR4 alleles of HLA-DR locus on chromosome six.