HLA-DR antigen distribution was determined by lymphotoxicity on total lymphocytes for locus A.B.C. antigens numbering 14,28 and 7 respectively, and by a search on B lymphocytes for the 12 antigens of locus DR. The normal population included 124 subjects typed for HLA-A.B.C. and 200 subjects typed for HLA-DR. The frequency of alleles was compared to that of the different groups of patients. Significant variations were evaluated by the X2 test, using Woolf's method; the P value obtained was multiplied by the number of antigens looked for (P corrected, or pc). No deviation in frequency was found with the HLA-A.B.C. antigens. Only the DR 4 antigen, present in 23% of the normal population, was increased in proportions that depended on clinical classification: 39.4% (Pc = 0.05) in all patients with giant cell arteritis: 27% (NS) in the 37 polymyalgia rheumatica patients with negative biopsy of the temporal artery; 46.8% (P 0.05) in the 62 patients with Horton's disease presenting either as clinical and histological temporal arteritis (26 cases; DR 4 = 38.5%; NS), or as clinical and/or histological temporal arteritis associated with polymyalgia rheumatica (36 cases; DR 4 = 52.8%; Pc less than 0.005). The frequency of DR 4 antigen in Horton's disease with typical temporal artery biopsy (37 cases) was 46% (Pc = 0.05).