Pneumonia complicates about half of all bone marrow transplantations, and in about a third of the cases no specific cause is identified. Although parainfluenza virus is a common cause of respiratory infection in normal children, its role in transplant recipients is unknown. We describe the incidence and clinical course of parainfluenza infection among the 1253 recipients of bone marrow transplants at our center from 1974 through 1990. We performed viral cultures on all such recipients who had manifestations of a viral infection or fever without apparent cause. Among the 1253 patients, we found 27 (2.2 percent) who had parainfluenza virus infection as demonstrated by culture (12 of 580 adults and 15 of 673 children). Eight of these patients had only upper respiratory tract involvement, all of whom had positive nasopharyngeal cultures. Of the remaining 19, 8 had symptoms of both upper and lower respiratory tract involvement, and 11 had only lower respiratory involvement, of whom only 6 had positive nasopharyngeal cultures. Four required bronchoalveolar lavage for diagnosis. A median of nine days elapsed from the onset of symptoms until the culture became positive, and overall only 33 of 118 cultures obtained were positive. Respiratory failure developed in 6 of the 19 patients with lower respiratory tract involvement, and all 6 died. Parainfluenza virus is a cause of serious lower respiratory tract involvement in both adults and children who undergo bone marrow transplantation. Given the insensitivity of current culturing techniques, it may be underdiagnosed. (N Engl J Med 1992;326:921–6.).