We have followed 106 recipients of allogeneic bone marrow transplantation (BMT), and observed 47 episodes of interstitial pneumonitis (IP) in 37 patients. Cytomegalovirus (CMV) pneumonitis was seen in 18 episodes in 18 patients, of whom 10 patients were diagnosed by bronchoalveolar lavage (BAL), 7 patients by autopsy, and 1 patient by sputum culture. There was one varicella-zoster virus pneumonitis diagnosed by autopsy. However, we could find no apparent etiology of pneumonitis in the other 18 patients. The median interval between BMT and episodes of IP was 74 days (range, 23-578 days) for CMV pneumonitis, and 180 days (range, 41-428 days) for idiopathic pneumonitis (p less than 0.05). BAL was more sensitive than transbronchial lung biopsy in the diagnosis of CMV pneumonitis, and CD4/CD8 ratio in BAL fluid was inverted in all episodes of CMV pneumonitis. Computed tomography (CT) scans were performed in 31 episodes in 26 patients, and all scans revealed abnormalities. CT scans were more sensitive than routine chest X-rays against micro- or small-nodular patterns, air bronchogram, and air alveologram (p less than 0.05). CT scan and arterial blood gas analysis were most useful and necessary in approaching the problem of pneumonitis in allogeneic marrow transplant patients. Based on these findings, when CMV pneumonitis is suspected, it is recommended to perform BAL for the final diagnosis of CMV pneumonitis.