Pulmonary hemorrhage is a relatively infrequent condition in infants that can result in significant morbidity and mortality. Its diagnosis is based on the identification of alveolar hemosiderin-laden macrophages (iron-staining white cells) from bronchoalveolar lavage or biopsy. A cluster of 10 cases of idiopathic pulmonary hemorrhaging (IPH) occurred during 1993 and 1994 at Rainbow Babies and Children's Hospital in Cleveland, Ohio. In the decade preceding these years, there had been only 3 cases of IPH at the same hospital. An examination of the circumstances that may have lead to pulmonary hemorrhage in these infants suggested a link between IPH and water damage in the patients' homes.