Interleukin-10 (IL-10) is a pleiotropic cytokine produced by type 2 helper cells (Th2), as well as by monocytes and macrophages, and normal and neoplastic B lymphocytes. It is highly homologous to an open reading frame of Epstein-Barr virus (EBV) called BCRF1, and EBV infection of B-cells up-regulates IL-10. IL-10 production has strong immunosuppressive effects via inhibition of Th1 type cytokines, including interferon gamma and interleukin-2. On B-cells, IL-10 has a potent stimulating effect, inducing proliferation and differentiation. Interestingly, in cell lines derived from B-cell lymphomas, IL-10 production has been found to be up-regulated, and it serves as an autocrine growth factor. In patients with non-Hodgkin's lymphoma (NHL), serum IL-10 levels are significantly increased when compared to normal individuals and NHL patients in remission. The prognostic significance of these increased levels vary according to the assay used. Both human IL-10 and viral IL-10 are increased, and when specific assays for human IL-10 are used, there seems to be no prognostic significance, whereas when the assay cross-reacts with viral IL-10, high levels correlate with poor prognosis. These results suggest that viral IL-10 might have some pathogenic role in NHL.