The Epstein-Barr virus load in the peripheral blood at the time of diagnosis of post-transplant lymphoproliferative disease (PTLD) is elevated 1000- to 10,000-fold compared to the level detected in normal latency. With the use of quantitative polymerase chain reaction (PCR), changes in the viral load over time can be measured with a two- to fourfold accuracy. This has allowed early detection of first-time infections and reactivations that may lead to PTLD and has provided an opportunity to intervene before symptomatic disease has occurred. Viral load monitoring has also been used to follow patients with PTLD and, along with other parameters, provided an assessment of the effectiveness of therapeutic protocols. Viral load monitoring has led to the discovery that at least two-thirds of transplant recipients become persistent viral load carriers. While the persistent load appears to be largely carried in latently infected memory B cells, more work is needed to clearly define this type of persistent infection and determine the risks associated with it. New diagnostic tests need to be developed to distinguish the persistent latent viral loads from viral loads that are likely to become symptomatic PTLD.