A registry of persons with the X-linked lymphoproliferative syndrome, which is characterized by marked susceptibility to diseases induced by the Epstein-Barr virus, has enrolled 161 patients within 44 kindreds. Fifty-seven percent of the males died of infectious mononucleosis, 29% developed acquired hypogammaglobulinemia, and 24% had malignant lymphoma. The mortality rate was 80%; 70% died by 10 years of age and 100% by 40 years. Thirty-two boys survive, most with malignant lymphoma, acquired hypogammaglobulinemia, or both. We hypothesized that the defective lymphoproliferative control locus on the X chromosome results in unregulated cytotoxic lymphocytic responses to the Epstein-Barr virus; hence, severe hepatitis and virus-associated hemophagocytic syndrome occur with the infectious mononucleosis phenotype. T-cell suppression of immunoglobulin secretion by B cells is responsible for acquired hypogammaglobulinemia. A sustained polyclonal B-cell proliferation probably converts to a monoclonal B-cell malignancy as a result of molecular alterations.