Patients with compromised immune function suffer a wide variety of lung insults. Infections are the most common causes of both acute and chronic lung diseases, but many noninfectious conditions affect the lungs. The clinical presentation of these noninfectious conditions often mimic infections, thus causing diagnostic dilemmas. The spectrum of noninfectious lung injury and response in the immunosuppressed host includes interstitial edema, interstitial fibrosis, diffuse idiopathic pneumonia, acute respiratory distress syndrome, and obliterative bronchiolitis. Alveolar hemorrhage may complicate any of these conditions. Lung injury in the immunosuppressed host is associated with a diversity of etiologies: sepsis, irradiation, graft rejection, reperfusion injury, graft-versus-host disease, and chemotherapeutic agents and other drug reactions. These injuries most often present as diffuse pulmonary infiltrates on chest radiograph. Establishing a specific diagnosis and etiology for the injury is often problematic. From a pragmatic standpoint, excluding the possibility of infection is the principal aim of diagnostic testing.