Organ transplant recipients have a higher risk of Kaposi sarcoma (KS). A quantitative real-time polymerase chain reaction assay was developed to evaluate KS-associated herpesvirus (KSHV) as a prognostic tool in transplant recipients with KS. Forty-three patients who developed KS after transplantation were included in a cross-sectional study to correlate virus load with transplantation or KS parameters. Seventeen patients (40%) had KSHV viremia (>100 copies/microg of DNA; median, 6067 copies/microg of DNA). Factors associated with these levels of viremia by univariate analysis were progression of KS (P=.00002), time from KS diagnosis (P=.0007), actual stage of KS (P=.006), initial stage of KS (P=.22), graft loss (P=.013), and time from transplantation (P=.0246). Disease progression remained associated with KSHV viremia in a multivariate analysis (P=.01). Thus, quantification of KSHV load in peripheral blood mononuclear cells could represent a useful tool for monitoring transplant recipients with KS.