Introduced in the early 1970sfor treatment of aplastic anemia and leukemia, bone marrow transplantation offers potentially effective treatment for a growing number of patients. The procedure is now recognized as therapeutic for an increasing number of diseases. Furthermore, advances in transplant immunobiology, supportive care, and prevention of graft-versus-host disease, coupled with the availability of suitable donors, make the technique both effective and feasible. In 1990 more than 5,500 patients received allogeneic marrow transplants from matched or partially matched family members, and more than 5,000 autologous transplant procedures were performed. The recent creation of the National Marrow Donor Program (NMDP), which currently lists more than 600,000 potential donors, has made identification of unrelated phenotypically HLA-identical donors possible for patients who do not have suitable donors among family members. It is estimated that more than 500 unrelated transplants will be performed in 1992. Despite these encouraging developments, transplantationrelated complications, especially those involving the lung, have limited the success of bone marrow transplantation. Interstitial pneumonitis accounts for more than 40% of transplantation-related deaths in most large series. Of these pneumonias, approximately half are noninfectious idiopathic pneumonia, herein referred to as idiopathic pneumonia syndrome (IPS). Although IPS is an important clinical entity, progress in understanding the pathogenesis of IPS has been limited. The lack of progress is partly due to different definitions of the disease, different diagnostic criteria, and the relatively small number of patients studied, at most centers. However, our understanding of related