Immune deficiency, especially to the Epstein-Barr virus, and increased susceptibility to fatal infectious mononucleosis, acquired agammoglobulinemia, and lymphoma are the cardinal features of the X-linked lymphoproliferative syndrome. Since the establishment of the XLP Registry in September, 1978, 59 affected males in seven unrelated kindreds were comprehensively studied. A spectrum of lymphoproliferative phenotypes was observed. Thirty-four patients (57%) died from infectious mononucleosis, eight (14%) had fatal infectious mononucleosis with lymphoma (immunoblastic sarcoma), nine (15%) had depressed immunity following EBV infection, and eight (14%) developed lymphoma. Several patients with XLP lacked EBV antibodies despite infection by EBV. The results of this study suggest that EBV can be an oncogenic agent in patients who are immune deficient with XLP.