This article discusses an investigation of two children with idiopathic pulmonary hemosiderosis (IPH) over a long period of time that included several cycles of the disease in each patient, each cycle consisting of clinical remission, a preacute or linking phase, and acute lung bleeding. The acute phase was divided into two parts; the preacute phase takes place 5 to 10 days before the onset of the acute alveolar hemorrhage. At the beginning of the preacute phase, there was neutropenia followed by increased blood eosinophilia. The lung function test showed gas trapping. Bronchoalveolar lavage during remission showed increased numbers of neutrophils; the histologic study suggested that the neutrophils are obligatory participants in the occurrence of lung bleeding. The finding of the preacute phase and the role of the neutrophils in the acute bleeding permitted development of a new hypothesis related to the physiopathology of IPH.