A prospective study of cytomegalovirus (CMV) infections has been carried out in 28 renal graft recipients. The protocol called for frequent blood and urine sampling during the first year after transplantation, but death or graft loss caused earlier termination in nearly half the patients. In this material 5/7 (71%) susceptible patients developed primary infections and 20/21 experienced a secondary infection (95%). Viruria was detected in 79% and viremia in 43%. The type of blood cell responsible for the viremic phase was studied by separating the blood cells on a density gradient. The polymorphonuclear cell fraction was the most common source of virus but virus could also be recovered from the mononuclear cell fraction. As some samples that were freeze-thawed repeatedly never yielded virus, it would appear that viable cells are needed for virus isolation. In both primary and secondary infections isolation of CMV from blood cells often preceded the isolation of CMV from urine. Among variables tested for a possible relationship to the occurrence of CMV viremia the only one to display such an association was the time at which rejection episodes occurred. In 19/28 such episodes recorded in 19 patients there was a temporal relationship to viremia (p less than 0.03). Seven of the patients experienced clinical symptoms suggestive of CMV infection as fever, cough, myalgia, arthralgia, chest pain and pneumonia. Laboratory signs included elevated amino acid transferase levels, leukopenia and thrombocytopenia and a specific anti-CMV antibody response.