Detection and semiquantitation of cytomegalovirus (CMV) DNA in plasma from 17 immunocompromised patients with CMV pneumonia diagnosed histopathologically, 15 CMV seropositive patients without CMV pneumonia and 24 CMV-seropositive healthy volunteers were evaluated, using the polymerase chain reaction (PCR). CMV DNA was detected in plasma from all of 17 patients with CMV pneumonia, from 1 of 15 patients without CMV disease, but from none of healthy volunteers. One patient without CMV disease exhibited positive CMV DNA by PCR 2 days before death. Plasma CMV DNA was negative at the time of admission in all patients, however, it became positive 1-28 days (mean, 14 days) before the onset of CMV pneumonia in 16 patients. The amount of viral DNA in plasma were 10(3) - 10(5) copies/ml (mean, 10 (4.0) copies/ml) when first detected by PCR. At the onset of CMV pneumonia, they were 10(4)-10(6)(mean, 10(5.3) copies/ml), and increased with disease progression and decreased with disease improvement because of treatment with antiviral agents. We succeeded in detection of CMV DNA in plasma before the development of CMV pneumonia, and showed the amount of viral DNA reflected the extent of active CMV pneumonia. Thus, PCR amplification of CMV DNA in plasma is a useful tool for early diagnosis and monitoring of immunocompromised patients.