The case histories of 72 subsequently treated patients - 44 with acute leukemia, 10 with chronic myeloid leukemia, 16 with severe aplastic anemia and 2 with neuroblastoma - were analyzed after bone marrow transplantation (BMT) with respect to pulmonary diseases. Thirty-eight patients suffered from a total of 51 pulmonary complications, which led to death in 20. Of 13 patients, 3 died of bacterial pneumonia, all of them during granulocytopenia; 2 of 6 patients died of fungal pneumonia and 2 out of 3 of a mixed bacterial-mycotic infection. Adult respiratory distress syndrome (ARDS) led to death in 2 patients. A granulocyte count under 500/microliter correlated significantly (P less than 0.002) with the fatal outcome of bacterial, fungal and ARDS pneumonia as well as with bronchitis. Viral pneumonia led to death in 8 of 9 patients; in each there was a significant correlation (P less than 0.05) with graft-versus-host disease (GvHD). Patients with repeated episodes of pulmonary illness had significantly more chronic GvHD (P less than 0.05); several of these patients displayed a reduction in helper T cells and an increase in suppressor T cells in the peripheral blood. The natural killer (NK) cells were reduced and the percentage of activated NK cell level lay between 6% and 69%. B-cells were absent or deficient. These findings explain in part the absence of specific antibody reactivity. Five of these patients also contracted GvHD-associated obstructive bronchiolitis, which did not respond to therapy. Pulmonary infiltrates of unknown origin (including idiopathic interstitial pneumonia) occurred in 8 of the patients (11.1%), with a fatal outcome in 3 patients. Significant changes (P less than 0.05) in lung function after BMT appeared in the form of reduced vital capacity (VC) increased residual volume (RV) and an increase in RV expressed as the percentage of total lung capacity. Pulmonary diseases were the most common complication and cause of death in our patients after BMT.