HLA-A, B, C and D typing was performed in 19 patients with primary Sjögren's syndrome (primary SS) and in 15 patients with rheumatoid arthritis (RA) and secondary Sjögren's syndrome (RA-SS). In the primary SS group, the frequency of HLA-Dw2 was increased (p less than 0.01; "corrected" p greater than 0.05) while the frequency of Dw3 was non-significantly increased. In the RA-SS patients, the frequency of HLA-Dw4 was increased to 84.6% (relative risk = 22.8; p less than 0.001) and the frequency of Dw2 was non-significantly decreased. An increased frequency of Dw2 in primary SS has not been reported before. To investigate if this observation was due to chance, a new series of 16 patients with primary SS were HLA typed. In this new group, the frequencies of both Dw2 and Dw3 were significantly increased, while the frequency of Dw4 was significantly decreased. In the whole group of primary SS patients, the frequencies of Dw2 were 56.3% (relative risk = 3.7; p less than 0.001). Dw3:50.0% (relative risk = 2.8; p less than 0.01) and Dw4: 6.3% (relative risk = 0.28; p less than 0.05). We conclude that genetic factors associated with the HLA-system are involved in the development of Sjögren's syndrome and that these genetic factors are different in primary and secondary Sjögren's syndrome. In primary Sjögren's syndrome, the association with both Dw2 and Dw3 might suggest a further heterogeneity of the syndrome.