Respiratory syncytial virus (RSV) has been demonstrated to be an important cause of life-threatening pneumonia in adult bone marrow transplant recipients; however, its role in other immunocom-promised adults has not been defined. We prospectively studied all adult patients with leukemia who were hospitalized at M. D. Anderson Cancer Center (Houston) during a 1-year period (November 1993 through October 1994). During a 19-week period when RSV was prevalent in the community, it was isolated from 9 (10%) of 87 patients with leukemia who developed an acute respiratory illness. In 6 (75%) of 8 patients with profound chemotherapy-induced myelosuppression, the RSV infection was complicated by pneumonia, with an 83% mortality rate. RSV appears to be an important cause of severe and often fatal pneumonia in myelosuppressed patients with leukemia.