The object of an immunohistochemical search for vascular protein deposits in temporal arteritis is to assess the diagnostic possibilities in cases which are clinically typical but unconfirmed by biopsy results. In a group of older patients with arteritis, however, vascular aging may give rise to intimal thickening and a broad-spectrum deposition of protein. In an inter- and intra-individual comparison of vascular segments with and without arteritis we, however, found a few protein markers in arteritis which are essentially different from those in vascular aging. The intimal thickening and immune reaction in 9 selected marker proteins were graded 0-2, using the tunica media as reference for both properties. Of the nine proteins studied, alpha-2-macroglobulin was significantly increased, not only in segments affected with arteritis, but also in unaffected segments from the same biopsy as compared with biopsies from patients not suffering from this disease. 79% of patients with biopsy-confirmed arteritis also showed a significantly elevated serum alpha-2-macroglobulin as compared to 27% of those having only changes attributed to aging. In conclusion, immunohistochemical demonstration of deposits in the arterial wall and elevated serum levels of alpha-2-macroglobulin substantiate the clinical suspicion of arteritis in the absence of histological and inflammatory changes.