Post-transplant lymphoproliferative disorders (PTLDs) are heterogeneous severe complications occurring in 1-10% of transplanted patients. In most cases, PTLDs are associated with Epstein-Barr virus (EBV) infection but, recently, some clinical studies have reported an increasing number of EBV-negative PTLDs. Several studies have emphasized the critical role of the early identification of patients at risk for PTLD, in prompting the adoption of either pre-emptive strategies or timely treatment. To this purpose, monitoring of EBV DNA load in peripheral blood mononuclear cells is considered to be a useful test. Moreover, recently, the role of interleukin (IL)-10 in EBV-related diseases has been remarked, and high levels of IL-10 have been detected in PTLD patients. In this study, both EBV load and IL-10 were monitored in 38 PTLD patients at diagnosis and during follow-up, as well as in a control group, in order to establish the diagnostic role of the two tests, their relationship with the different PTLD subsets (EBV-positive and EBV-negative) and their behaviour during treatment. Results of our study suggest that the usefulness of IL-10 assay for early diagnosis of PTLD is similar to that of EBV load quantification, and its clinical diagnostic value is lower in EBV-negative than in EBV-positive PTLDs.