Three children developed severe respiratory distress at days +12, +11, and +11 following allogeneic bone marrow transplantation from donors. The first child was a 13-year-old Hispanic boy transplanted in relapse of Philadelphia chromosome-positive acute lymphoblastic leukemia (ALL). At day -14, a bronchial washing done for a streaky pulmonary infiltrate was negative for acid-fast bacilli. Miliary tuberculosis was discovered at postmortem examination. A second child, transplanted in remission of null-cell ALL, developed severe hypoxia and hypercarbia on day +11 but recovered fully following prolonged mechanical ventilation. An open-lung biopsy showed a pattern of nonspecific, diffuse alveolar damage compatible with respiratory distress syndrome. The third child was transplanted in remission of B-cell ALL and developed fatal fungal and cytomegalovirus pneumonia on day +12. In these latter two cases, it is likely that open-lung biopsy would have missed the diagnosis because of the uneven pulmonary involvement and multiple etiologies observed. All three children received cyclosporine, granulocyte transfusions, and multiple antimicrobials, including amphotericin B. Hyperfractioned total-body irradiation with lung shielding was used in the latter two patients.