Serological tests for cytomegalovirus (CMV) after cardiac transplantation were performed at six to eight-week intervals on 26 patients (3 females and 23 males; mean age 46 [15-62] years) with angiographic or ultimately autopsy evidence of coronary atherosclerosis (group 1) and 24 patients (5 females and 19 males; mean age 45 [25-56] years) without coronary disease in the transplanted heart. A positive result meant an at least fourfold increase in CMV IgG titre, demonstration of CMV IgM or direct viral isolation from blood or other body fluid. In 20 patients of group 1 (77%) a CMV infection had occurred after the transplantation, but in only six patients (25%) in the group 2 (P less than 0.0001). These results are interpreted as demonstrating a relationship between CMV infection and rapidly progressive coronary atherosclerosis after cardiac transplantation.