Cytomegalovirus (CMV) titer in blood seems to be the principal determinant of clinical symptoms in immunosuppressed patients. We have developed an assay for quantitation of CMV DNA in serum. The assay requires the coamplification by polymerase chain reaction (PCR) of extracted serum DNA with 1000 molecules of mutated internal standard DNA, and then an ELISA detection system. We examined 133 paired buffy coats and sera from 15 patients with symptomatic infection. Sera were examined by quantitative PCR, and buffy coats were examined by qualitative PCR (with a detection threshold of approximately 40 copies per 150,000 cells). Serum viral titers peaked during the seventh week after transplant (median day 40, range 26-58) at about the time of symptom onset. Mean viral titer measured during the seventh week was 1.2 x 10(5) copies per milliliter of serum (standard error 6.5 x 10(4). Buffy-coat PCR results were generally concordant with results of serum PCR (overall concordance 103/133=77.4%). Serum CMV titer fell, as symptoms resolved with reduction of immunosuppression and specific antiviral therapy. High titers and poor response to antiviral therapy were observed in the context of excessive immunosuppression and bacterial sepsis. Measurement of serum CMV titer may be useful for the management of immunosuppressed transplant recipients, and provides a tool for the better understanding of factors that enhance or inhibit viral replication.