Forty-three patients with well-documented giant cell arteritis (GCA) were studied clinically and with HLA typing. All patients were over age 40. Twenty-two of the patients had coexistent polymyalgia rheumatica (PMR). No association with class I HLA antigens was detected. When compared with HLA findings in 243 healthy controls, HLA-DR4 was increased in patients with GCA and PMR, but not in patients with GCA alone. Pooled analysis of data from 4 published papers confirmed the association of DR4 with GCA and PMR (P much less than 0.00001). Patients with PMR but negative findings of temporal artery biopsies also showed an increased frequency of HLA-DR4, but this did not reach statistical significance, probably because of the small number of subjects. It is likely that the increased frequency of DR4 in GCA patients is related to its association with PMR.