Electron microscopic examination of a lung biopsy obtained from a child in the active stage of idiopathic pulmonary hemosiderosis disclosed prominent changes in the basement membrane of alveolar capillaries. Review of the pertinent literature showed that a structural defect of lung capillary walls in this disease has been proposed in the past mainly on theoretic grounds but not adequately documented at the ultrastructural level. Concentric multilayering of pulmonary capillary basal lamina, one of the changes observed in our material, is a rare phenomenon in this location, although one that has been well described in extrapulmonary microcirculatory beds and is probably a nonspecific reaction to vascular injury. The conceptual difficulties attending a definition of idiopathic pulmonary hemosiderosis are breifly discussed.