Eighteen patients underwent allogeneic bone marrow transplantation (allo. BMT) during the period May, 1991 to December, 1992 in the Center for Adult Diseases, Osaka. They were monitored for cytomegalovirus (CMV) antigenemia and arterial oxygen saturation (SaO2). More than 10 antigen-positive cells per 50,000 polymorphonuclear leukocytes were detected in five of 18 patients. Three of these 5 patients developed CMV pneumonia several weeks after the first detection of more than 10 positive cells. Six of 18 patients developed interstitial pneumonia (IP) (3 CMV pneumonia and 3 idiopathic IP). SaO2 decreased less than 95% several days before the development of IP in 3 of these 6 patients (2 of CMV pneumonia and 1 of idiopathic IP). CMV antigenemia assay and SaO2 assay were thus both considered to be useful for the early detection or prediction of development of CMV pneumonia.