A prospective serologic study was conducted to determine the incidence of primary Cytomegalovirus (CMV) infection after a renal transplant and to evaluate the consequences on the graft. Serologic tests using the indirect hemagglutination technique with CMV-sensitized erythrocytes were performed serially over 6 years in 370 children initially awaiting renal transplantation. 211 children received a graft during the study period. Transplantation was followed by primary CMV infection in 48 (35.3%) of the 136 children with negative serologic tests at the time of the procedure. The percentage of graft failures is identical in the 75 children with positive tests before transplantation and in the 88 children with negative tests before and after transplantation (24 and 23% respectively). Conversely, only 6 (12.5%) graft failures were recorded among the 48 children who experienced primary CMV infection after transplantation. It can be speculated that these results are due to the immunosuppressive effect of CMV infection.