This report deals with 81 pulmonary episodes occurring in 130 consecutive patients who underwent allogeneic bone marrow transplantation for hematologic malignancy in the same unit over a 5-year period. These episodes observed in 69/130 patients (53%) were mostly of infectious origin, and were investigated by bronchoalveolar lavage (BAL). The main causes of pneumonia were: cytomegalovirus (CMV) (n = 25), bacterial pneumonia (n = 17), invasive aspergillosis (n = 11) and pulmonary hemorrhage (n = 9). The overall mortality due to or associated with pneumonia was 26/130 (20%). Graft-versus-host disease clearly increased the incidence of infectious pneumonia and the mortality due to or associated with pneumonia. Granulocyte transfusions did not influence the incidence of CMV pneumonitis. The main causes and risk factors for pneumonia are discussed. The role of BAL as a noninvasive procedure is stressed.