Sera from 74 patients with polymyalgia rheumatica or giant cell arteritis or both were tested for immune complexes by using the Raji cell radioimmunoassay. Levels in patients with active disease were higher than in patients whose disease had become inactive. There was no difference in levels of immune complex-like materials between patients with polymyalgia rheumatica alone and those with giant cell arteritis. Density gradient analysis of one serum showed immune complex-like materials mainly in the 19S region. Immune complexes may be important in the pathogenesis of these conditions.