Of 386 patients with allogeneic bone marrow transplants (BMT) treated during a 9-year interval, 166 developed interstitial pneumonitis (IP). Idiopathic and cytomegalovirus (CMV) IP constituted 90% of the 113 cases in which tissue was examined. Risk factors for IP overall were acute graft-versus-host disease (AGVHD), remote transplant date, the diagnosis of leukemia, and GVHD prophylaxis with agents other than cyclosporine. Risk factors for CMV IP were pre-transplant CMV seropositivity, CMV excretion, age greater than 10 years, AGVHD, GVHD prophylaxis with agents other than cyclosporine, and a remote transplant date. Patients transplanted for aplastic anemia were at lower risk for idiopathic IP than those transplanted for leukemia. The incidence of IP in patients given busulfan plus cyclophosphamide was equivalent to that in patients receiving cyclophosphamide plus total body irradiation. The incidence of idiopathic IP remained constant over this 9-year period while CMV IP declined significantly.