Idiopathic pulmonary hemosiderosis (IPH) is an infrequent condition whose severe and unpredictable prognosis justifies extensive etiologic investigations. We have assembled 12 cases of IPH (nine children), three adults). In seven patients, pulmonary bleeding was demonstrated upon bronchoalveolar bleeding was demonstrated upon bronchoalveolar lavage (BAL) that recovered numerous hemosiderin-laden alveolar macrophages (Golde index: 247 +/- 53). All 12 patients underwent a surgical lung biopsy. Light microscopy studies showed hemosiderosis, often with lymphoid hyperplasia (n = 11), and occasionally with large germinal centers (n = 4), interstitial mastocytes (n = 7), ferric tattoo of the elastic network (n = 4), and a variable degree of interstitial necrosis (n = 4). Ultrastructural studies were performed in six cases and showed swelling of capillary endothelial cells (n = 5), interruptions in the endothelium (n = 3), tattoo of basement membranes (BMs) and elastic tissue (n = 3), intracapillary platelet aggregates (n = 2), and focal thickening of capillary BMs (n = 2). Four biopsies were studied using immunofluorescence (IF): no deposits of immune complexes were found. Indirect IF assays for antiglomerular and alveolar BMs was performed in two patients and was negative in both.