Bone marrow transplantation (BMT) for hematologic disorders is potentially curative in selected persons. These patients may be immunocompromised for months after engraftment as a consequence of chemotherapy, irradiation, acute and chronic graft-vs-host disease (GVHD), and maturing recipient marrow. Pulmonary complications commonly occur during the early and late periods after BMT and are associated with significant morbidity and mortality. The leading early-onset complication is interstitial pneumonitis, most commonly associated with cytomegalovirus infection but also related to possible toxicities from chemotherapy and irradiation. Major late-onset problems include bacterial sinopulmonary infections and obstructive airway disease thought to be associated with chronic GVHD. The exact mechanisms of lung injury are probably quite complex, and unfortunately, often cause irreversible pulmonary disease, even in the patient who has had successful transplantation. Antimicrobial prophylaxis, modified chemotherapy and irradiation dosages, and antiviral immunization have been shown to reduce the incidence of early-onset pulmonary problems. Early recognition and treatment of late-onset problems will, it is hoped, minimize respiratory limitations.