Cytomegalovirus (CMV) infection was established by virologic or serologic studies or both in 30 of 41 renal allograft recipients (73%) who survived one or more months after transplantation. The infection occurred in 22 of the 24 patients (91%) with serologic evidence of exposure to CMV before transplantation, but in only eight of 17 (47%) who had no demonstrable serum antibody at the time of the surgical procedure. The data strongly suggest that active CMV infection in renal graft recipients can result either from the activation of "latent" virus or from exposure to the virus after transplantation. The occurrence of the infection did not correlate with a well-defined clinical syndrome even though generalized cytomegalic inclusion disease was found in some patients at autopsy.