Background. Diffuse pulmonary infiltrates is the major complication and cause of mortality after bone marrow transplantation. We analyzed the etiologies and prognostic factors in bone marrow recipients with diffuse pulmonary infiltrates and assessed the role of open lung biopsy in managing this complication. Methods. Medical records of patients with diffuse pulmonary infiltrates after bone marrow transplantation were reviewed. Possible prognostic factors were analyzed by multivariate logistic regression. Results. Sixty-eight (20%) of 341 bone marrow recipients had diffuse pulmonary infiltrates and 34 died. Thirty-five underwent open lung biopsy, resulting in therapeutic changes in 22 (63%) and clinical improvement in 16 (46%). The leading diagnoses were idiopathic interstitial pneumonitis ( 40%) and cytomegalovirus pneumonitis (20%). Cytomegalovirus pneumonitis caused radiographically observable interstitial infiltrates exclusively and was frequently associated with hepatitis. Idiopathic interstitial pneumonitis resulted in either diffuse ground-glass opacity or interstitial infiltrates. Three (9%) patients had miliary tuberculosis. Respiratory failure (p