Sixty-four temporal arteries were studied. 36 were from patients with clinically active temporal arteritis or polymyalgia rheumatica; 22 showed histological changes of temporal arteritis, 12 of which were in an active stage. 28 arteries, none of which showed histological changes, were taken at necropsy or from patients with unrelated disease. Extracellular immunoglobulin and complement deposition was seen in the artery biopsies showing active arteries and in 1 of the 10 biopsies with inactive arteritis. There was no immunoglobulin or complement deposition in the 14 patients with clinically active temporal arteritis and/or polymyalgia rheumatica, but with a normal artery biopsy. Patients with clinically active temporal arteritis were more likely to have a positive biopsy. Our results support the suggestion that the immune deposition is concurrent with an active histologically proven arteritis. Immunofluorescent examination does not appear to be a better diagnostic test than histological examination.