Pneumonia occurs in up to 50% of patients after bone marrow transplant and is the main cause of mortality. The patient may be predisposed to pulmonary complications by previous treatment and infections, by transplant conditioning and graft-versus-host disease prophylaxis regimens, and by prolonged severe immune-suppression. The period of greatest risk is within 3 months of transplant when focal or diffuse interstitial pneumonias may occur. The most common infectious etiologies are Aspergillus or other fungi and cytomegalovirus. Idiopathic, noninfectious, interstitial pneumonia occurring in this same period may represent treatment toxicity. Both fungal and cytomegalovirus infections have been associated with high mortality, but there has been significant success in treating cytomegalovirus with the combination of ganciclovir and immunoglobulin in recent years. Late bacterial pneumonia may occur more than 6 months after transplant because of prolonged immunedeficiency and functional asplenia. Prophylactic therapy to prevent the pneumonias anticipated after transplant should be an essential part of the care of the bone marrow transplant patient.