In 10 of 14 patients with an active cytomegalovirus (CMV) infection, distinctive large cells (35-45 microns in diameter) were present in the peripheral blood. Morphologically these cells closely resembled the classic cytomegalic inclusion cells, generally regarded as a diagnostic hallmark of CMV infection. Moreover, these cells were shown to express CMV antigens belonging to all three stages of the viral replication cycle, indicating a productive CMV infection. In addition, immunologic staining with monoclonal antibodies directed against cell differentiation and marker proteins showed that these circulating cytomegalic cells were of endothelial origin. The presence of CMV-infected endothelial cells in the peripheral blood of patients with an active CMV infection indicates that such an infection might be accompanied by widespread occult vascular damage.