We analyzed serum from six patients with pulmonary eosinophilic granuloma for the presence of circulating immune complexes. These levels were correlated with the pulmonary histopathology and immunofluorescence findings. Levels of circulating immune complexes were elevated in five subjects. All of the subjects had an active cellular histology. Immunofluorescent studies showed granular deposits of IgG and complement (C3) in alveolar walls and blood vessels in all five patients. One subject had no detectable circulating immune complexes and showed a predominantly fibrotic pattern by light microscopy. In addition, immunofluorescence in this patient also showed no immunoglobulin or complement deposition within the lung tissue. These findings show that circulating immune complexlike activity is present in patients with cellular disease and suggest that their formation or deposition, or both, may contribute to the pathogenesis of pulmonary eosinophilic granuloma.