In order to evaluate the pathogenic role of human cytomegalovirus(CMV) infection in human vascular disease, we first examined the role of CMV immediate early gene (CMV-IE) expression in vascular smooth muscle cell (VSMC) proliferation. The in vitro IE gene transfer stimulated VSMC proliferation. The in vivo IE gene transfer showed neointimal thickening while the control arteries did not. In the wall of "so-called" inflammatory abdominal aortic aneurysm (IAAA), CMV infected cells were more frequently encountered than in that of AA and control cases. CMV infected cells were largely identified as macrophages or fibroblasts, and these cells frequently expressed CMV-IE gene. These findings thus suggest that the persistent expression of CMV-IE gene in the vessel wall may play a role in the vascular cellular responses, including progression of atherosclerosis or vasculitis, in vivo.