A 21-year-old man had chronic renal failure and received a renal transplant. We used the polymerase chain reaction technique to detect cytomegalovirus (CMV) DNA in serum, blood, and urine for the diagnosis of CMV infection after the renal transplantation. CMV DNA in the serum was detected by polymerase chain reaction only during the active stage, while CMV DNA in urine and blood was detected even during the silent stage of CMV infection. It is suggested that the use of the polymerase chain reaction in serum would be useful for the rapid diagnosis of active CMV infection.