Complement evaluation was performed in two patients with active eosinophilic pneumonia and in one in remission, to determine the role of complement activation in the pathogenesis of this disorder. All three had cough, dyspnea, malaise, and blood eosinophilia; two patients also had pyrexia. In all 3 cases the pulmonary eosinophilic infiltrates (radiographic findings) and symptoms responded rapidly to steroid administration. The two patients with active eosinophilic pneumonia showed elevated CR3 but reduced FcrR on the PMN before and during steroid administration. In contrast PMN from four patients with bronchial asthma exhibited slightly elevated expression of both CR3 and FcrR during their asthma attack. It is suggested that clinical symptoms disappear soon after the beginning of steroid but changes of complement receptors on PMN may last for longer periods. On the basis of the combined results, this study indicates that estimation of complement activation may provide a useful indicator for disease activity in patients with eosinophilic pneumonia of unknown etiology.