During a 2-year period, 157 consecutive episodes of respiratory virus infections that occurred in 130 patients with upper or lower respiratory tract infection were analyzed for respiratory viruses. A respiratory virus was identified in 75 episodes (48%), and several viruses were found in 13 episodes: there were a total of 56 influenza A virus infections, 14 respiratory syncytial virus infections, 8 adenovirus infections, 8 infections with parainfluenza virus types 1 or 3, and 7 enterovirus infections. On multivariate analysis, the only variable that predicted progression to pneumonia in patients with an upper respiratory tract infection was the presence of respiratory syncytial virus, whereas lymphocytopenia had a nonsignificant trend. Also, among the 38 patients who had pneumonia at any time during the episode, both respiratory syncytial virus and lymphocytopenia were commonly found. For both epidemiological and therapeutic considerations, frequent screening for respiratory viruses should be incorporated into the routine diagnostic study of patients with hematologic malignancies.