Circulating cytomegalovirus (CMV)-infected endothelial cells (CCIC) have been found in immunocompromised patients with CMV disease and have been associated with disease severity. The frequency of CCIC in marrow transplant recipients was studied to determine its use for distinguishing between CMV pneumonia and asymptomatic CMV infection. CCIC were found in 13 (81%) of 16 patients with CMV pneumonia (5/6 without copathogen; 7/10 with copathogen) and in 10 (50%) of 20 patients with asymptomatic CMV antigenemia at different levels. There was no statistically significant association for the incidence nor the quantitative level of CCIC in patients with asymptomatic CMV antigenemia, CMV pneumonia with or without copathogen, or different levels of CMV virus load measured by antigenemia. The high incidence and quantitative level of CCIC in patients without clinical disease suggests a role for CCIC in CMV dissemination rather than its being a specific marker of organ or tissue damage after marrow transplantation.