Lung disease in patients with severe combined immune deficiency (SCID) undergoing bone marrow transplantation (BMT) is most commonly caused by infection. Noninfectious episodes of pulmonary disease following BMT are more frequently encountered in patients with hematologic disorders or malignancy and are probably related to ablation therapy or graft-versus-host disease (GVHD). In contrast, patients with SCID do not receive chemotherapy before an HLA-identical allogeneic BMT and they do not suffer significant GVHD. We report a patient who developed severe lung disease during the period of rapid engraftment following an HLA-identical allogeneic bone marrow transplantation. Lung biopsy showed dense lymphocytic infiltrates in the alveolar septae and no evidence of infection. Following the idea that the acute recruitment of engrafted lymphocytes may have contributed to or caused the pulmonary disease, we have attempted to suppress cellular immunity by administering high-dose methylprednisolone. The patient's lung disease rapidly improved and eventually completely resolved.