Sjögren's syndrome is associated with a serologically defined histocompatibility antigen of the HLA-B locus, HLA-B8. Another closely linked locus, HLA-D, determines lymphocyte-defined cell-surface antigens. In laboratory animals such antigens are closely linked to immune response genes. To determine whether Sjögren's syndrome is primarily associated with an HLA-D antigent or with HLA-A and HLA-B antigens as well as HLA-Dw3, which is known to be in linkage disequilibrium with HLA-B8. We found the primary association of association of Sjögren's syndrome to be with HLA-Dw3, which we observed in 84 percent of patients with Sjögren's syndrome in the absence of rheumatoid arthritis as compared to 24 percent in controls (P less than 0.00001). The frequency of HLA-B8 and HLA-Dw3 in patients with both Sjögren's syndrome and rheumatoid arthritis did not differ from that in controls. Patients with Sjögren's syndrome and rheumatoid arthritis comprise genetically distinct groups.