We report immunological studies performed in 19 sera from 4 children with idiopathic pulmonary hemosiderosis (IPH). Circulating C1q binding immune complexes (IC) were found in 3/4 of the patients during the acute phase. One child showed IC in 4 consecutive crises but they disappeared very rapidly (within 36 hours). Only one serum was positive after acute crisis. In one patient who had 2 episodes. IC were never detected. Precipitation with 2% polyethylene glycol did not correlate with the C1q binding assay. Pulmonary biopsy was available in just one patient and neither immunoglobulins nor complement were found by immunofluorescence. IgA, IgM, IgE, C4 or B factor abnormalities were only occasionally seen. Antinuclear, anti-reticulin and anti-alveolar basement membrane antibody test were always negative. Precipitins against cow's milk proteins were not detected. Seric IC could be primary or secondary to macrophage blockade by hemosiderin. Even in this case, it is well known that circulating IC can perform some immunological actions by interacting with cell receptors or releasing active mediators. In the future, the possibility that IC could contribute to IPH pathogenesis, or modify the treatment response must be taken into account.