Kaposi's sarcoma-associated herpesvirus (KSHV) is found in Kaposi's sarcoma (KS), multicentric Castleman's disease, and primary effusion lymphomas. To prospectively evaluate KSHV load as a biomarker for KS clinical status and prognosis in a cohort of men with AIDS-related KS, 2 quantitative polymerase chain reaction (PCR) assays were developed and tested to determine KSHV peripheral blood mononuclear cell (PBMC) virus loads. Most patients (13/15) with good-prognosis KS had ⩽ 1.5 log KSHV copies/10⁵ PBMC by both quantitative competitive (QC) and real-time Applied Biosystems (ABI) PCR. Both assays provided 94% specificity for identifying the 16 patients without KS progression during 20 months of follow-up. QC-PCR and ABI-PCR exhibited 100% and 80% levels of diagnostic sensitivity, respectively, for identifying the 5 patients whose KS progressed. Neither dichotomized human immunodeficiency virus loads nor dichotomized CD4 counts predicted either KS progression or KS clinical stage (all positive predictive values < 30%). These results are evidence that the quantity of circulating KSHV in KS patients is biologically meaningful and is measurable with sufficient accuracy to provide clinically useful information.