There are indications that the cytokine interleukin (IL)-10 has a regulatory role in Epstein-Barr virus (EBV)-induced infections. Because the human IL-10 gene demonstrates polymorphism resulting in interindividual differences in cytokine production, the frequencies of the alleles defined by the base exchange polymorphism at the position -1082 (allele 1=G, allele 2=A) were analyzed in EBV-seronegative adults, seropositive adults, and in patients hospitalized because of a severe EBV infection. The frequencies of allele 1 were 0.80, 0.46, and 0.29, respectively. Because this allele is associated with a high IL-10-producing capability, these data suggest that high IL-10 levels protect against EBV infection and, conversely, that low IL-10-producing capability makes individuals more susceptible to a severe EBV infection.