Bronchoalveolar lavage material from 54 immunocompromised patients with interstitial pneumonia was examined by immunofluorescence with cytomegalovirus-specific monoclonal antibodies. Twelve patients (22%) had cytomegalovirus detected in their lavaged cells, and 9 of these patients (17%) had proven cytomegalovirus pneumonitis. This assay detected all samples with cytomegalovirus when the virus was detected by established methods either at the time of lavage or after any other procedure in the subsequent 2 months; that is, it had a sensitivity of 100%. Cytomegalovirus could be detected within 3 hours of the lavage, and a clear correlation was seen between the number of fluorescent cells and the presence of cytomegalovirus pneumonia. All 9 patients with pneumonitis had more than 0.5% fluorescent cells, whereas the 3 patients in whom cytomegalovirus was detected without pneumonia had significantly fewer fluorescent cells. This method provides a sensitive, rapid, and quantifiable system for detection of cytomegalovirus, facilitating the early diagnosis and treatment of cytomegalovirus pneumonia.