Quantification of Epstein-Barr virus (EBV) in peripheral blood is important for the diagnosis and management of serious EBV diseases, including posttransplant lymphoproliferative disorder. A variety of PCR-based methods are currently in use; however, there is little information on their comparability. This study assessed the relative performance of different quantitative assays. A multicenter comparative study was performed at eight sites using three panels consisting of serial dilutions of quantified EBV DNA and extracts from a total of 19 whole-blood specimens. Samples were distributed and tested blindly. Instrumentation, probe chemistries, amplification targets, and other test-related aspects varied considerably between laboratories. Each laboratory's calibration curve indicated strong evidence of a consistent log-linear relationship between viral load and cycle threshold, suggesting that intralaboratory tracking of a given patient would yield similar relative quantitative trends among the participating test sites. There was strong concordance among laboratories with respect to qualitative test results; however, marked quantitative discordance was seen. For most samples, the across-laboratory interquartile range of the reported viral load (in copies/microl) was roughly 0.6 log-units, and for one sample the overall range was approximately 4.2 log-units. While intralaboratory tracking of patients may yield similar results, these data indicate a need for caution when attempting to compare clinical results obtained at different institutions and suggest the potential value to be gained by more standardized testing methodology.