Pulmonary toxicity occurs in approximately 10 to 50% of patients undergoing bone marrow transplantation (BMT). Bacterial pneumonia very commonly affects patients within the first 6 months post-BMT. Etiologic factors include neutropenia and the presence of graft-versus-host disease (GVHD). Pulmonary fungal infections, due to candida and aspergillus, may develop in 16% of patients receiving BMT, with a high mortality rate, being about 80%. A prolonged neutropenia as well as GVHD and associated immunosuppressive treatments are important factors in predisposing a patient to develop fungal pneumonitis. Interstitial pneumonitis occurs in 10-40% of patients; herpes viruses are the most commonly documented cause, with cytomegalovirus (CMV) being the most common pathogen. No causative organism is identified in up to 60% of the cases. It is likely that some of these cases may result from drug or radiation toxicity. Lung shielding and fractionation of the dose have decreased the incidence of interstitial pneumonitis to less than 5%. Patients with GVHD are predisposed to lung infections because of the immunosuppression that accompanies GVHD and its treatment. In addition, GVHD itself appears to have a direct effect on pulmonary epithelium. Cultural and serologic studies as well as radiographic investigations and other diagnostic procedures (ie bronchoalveolar lavage) are needed for appropriate management of pulmonary complications.