To determine the incidence and symptomatology of cytomegalovirus (CMV) infection in renal transplant recipients, we followed 40 patients prospectively for 5 to 24 months after operation. As judged by the serological change and virus isolation from blood and/or urine, the infection rate for the entire group was 82% (33 of 40 patients). Two types of infection were recognized: primary infection that occurred in 3 patients, and reactivation infection that occurred in 30 patients. Twenty-one patients were asymptomatic and 12 were mildly symptomatic. Four had mild hepatic dysfunction; two had the CMV mononucleosis syndrome associated with mild to moderate deterioration of renal function. Two patients had hyperglycemia. One patient who reached a CMV antibody titer of 1:32,768 was totally asymptomatic other than fever of short duration. CMV-induced pneumonitis or retinitis was not noted, and there was no mortality from CMV infection in our patients. Factors which may be associated with the absence of severe manifestations of active CMV infection in these patients are discussed.