The clinical course of parainfluenza virus infection occurring in 8 of 265 (3%) adult bone marrow transplant recipients during 1991 was reviewed. Parainfluenza virus type 3 was isolated from all eight patients. The clinical course ranged from self-limited upper respiratory tract infections (2 patients) to severe lower respiratory tract disease (6 patients) associated with a 50% mortality. This study highlights the important role of community respiratory viruses such as parainfluenza virus in the etiology of pneumonia in immunocompromised adults.