Reactivation of infection with the cytomegalovirus (CMV) in renal transplant recipients may cause significant morbidity and mortality. To evaluate factors associated with activation of CMV replication, we followed prospectively a group of 68 renal transplant recipients for 12 months. The control group consisted of healthy blood donors (n = 37). Sera were collected periodically from these patients and analyzed for the presence of specific anti-CMV antibodies. Enzyme-linked immunoassay based on recombinant CMV proteins was used to detect the following antibody specificities: IgG and IgM. During the first year after transplantation, reactivation of CMV occurred in 48 recipients (70.6%). Detailed analysis did not show any association of reactivation with the type of basic immunosuppressive therapy, prophylactic or therapeutic use of anti-lymphocyte antibodies, as well as occurrence of acute rejection episodes. There was a borderline association (P=0.068) between the presence of CMV infection and EBV reactivation. In conclusion, results of our study suggest that CMV infection may represent a factor activating EBV replication.