In a retrospective analysis, serial lung function tests from 81 patients receiving bleomycin were studied to determine the accuracy of carbon monoxide diffusing capacity (DLCO) as a predictor of clinically significant bleomycin lung. Six of 81 patients developed clinically significant bleomycin lung, and the DLCO predicted its development in only one patient (sensitivity, one of six patients; 16.7%). Respiratory symptoms and chest x-ray abnormalities were the earliest manifestations in the other five patients. Seventy-five of 81 patients did not develop clinically significant bleomycin lung, and 12 of these had major falls (greater than or equal to 35% pretreatment level) in DLCO (specificity, 63 of 75 patients; 84.0%). In eight patients, bleomycin was continued after a major fall in DLCO, and none developed clinically significant lung toxicity. In this study, the DLCO failed to predict the development of serious bleomycin lung toxicity in all but one case. Furthermore, in some patients, it would appear that bleomycin may be stopped inappropriately after low DLCO measurements. It is important to monitor for respiratory symptoms and chest x-ray abnormalities during bleomycin treatment as these will be the earliest signs of lung toxicity in most cases.