The nested Polymerase Chaain Reaction (PCR) was compared with virus isolation and serology to establish which is the best method for the diagnosis of active cytomegalovirus (CMV) infection. Samples of blood leucocytes, urine and throat washings from immunosuppressed patients and patients with congenitally acquired CMV infection, and well as from healthy persons were examined with PCR. CMV DNA was detected in all samples from which CMV could be isolated, but not from any sample from healthy adults, whether CMV seropositive or CMV seronegative. In contrast to the findings in healthy persons, CMV genomes were frequently detected in urine and throat washings from immunosuppressed, CMV-seropositive patients without symptoms of CMV infection. The appearance of CMV genomes in blood cells in immunosuppressed CMV-seronegative patients may be the first sign of primary CMV infection. Congenital CMV infection could be rapidly and safely diagnosed when urine samples were examined by PCR. Nested PCR is a valuable tool for the diagnosis of active CMV infection, when selected materials are used.