IgG anticardiolipin antibodies (aCL) were found in a patient with Takayasu's arteritis and then in 11 of 22 patients with temporal arteritis (TA) studied prospectively. In comparison IgG aCL were found in only one of 18 elderly controls with rheumatoid arthritis and one of 47 healthy elderly controls. In TA IgG aCL were more frequent in those with acute disease (8/12), in those with an accompanying history of polymyalgia rheumatica (9/13), p less than 0.05); and levels normalized with corticosteroid treatment. Antineutrophil cytoplasmic antibodies (c-ANCA) were present in one patient with TA who had no systemic disease.