We determined circulating immune complex levels and their correlation with pulmonary histopathology, immunofluorescence and steroid responsiveness in 24 patients with idiopathic interstitial pneumonias. Levels were elevated in all but three of 16 patients with cellular disease, but in none of eight with diffuse fibrosis (P less than 0.001). Granular deposition of IgG, usually with elevated levels, but in only 11 per cent of those with normal levels (P less than 0.001). The radiographic and physiologic response to corticosteroid therapy was better in patients with initially elevated levels than in those with normal levels (P less than 0.03). Circulating immune complexex are present in patients with cellular idiopathic interstitial pneumonias, have a pathogentic role in this disease and may identify a patient population that is potentially steroid responsive.