Six patients developing cytomegalovirus infection following renal transplantation were studied to evaluate the type of infection that might occur. All developed virological or serological evidence of infection, while two developed symptomatic illness. A syndrome resembling mononucleosis was evident in one, while a hepatitis was seen in the other. Both patients were viremic during their illness and from two weeks to two months after they returned to normal health. A third patient showing no evidence of a viral illness was viremic. Cytomegalovirus was isolated from the erythrocyte layer of all three patients, but less regularly from the leukocytes, plasma, and serum. High-titer complement-fixing antibody was present during the viremic periods, and immunosuppression appeared to have no effect on the specific antibody response and little on semiquantitatively measured lgM, lgG, and lgA levels.