A quantitative competitive polymerase chain reaction (PCR) assay for human herpesvirus 8 (HHV8) was developed. The assay uses a competitive internal control sequence which differs from the wild type sequence by the presence of an EcoRI restriction endonuclease site. The quantitative method was validated by co-amplifying known amounts of control sequence DNA and wild type DNA, and shown to accurately quantify HHV8 within the range of 10(1)-10(6) genome copies (R = 0.998). The assay was used to quantify HHV8 in sequential blood samples of a renal transplant patient diagnosed with Kaposi's sarcoma (KS). A peak viral load of 28320 genomes/ml blood was present at diagnosis of KS, which reduced to 13680 genomes after 2 days and was undetectable 4 months later. Control of HHV8 replication in this patient parallelled the cessation of immunosuppressive therapy.