Patients with X‐linked lymphoproliferative (XLP) disease are characterized by extreme vulnerability to Epstein‐Barr virus (EBV). Following infection with EBV, affected males develop fatal infectious mononucleosis (IM), hypogammaglobulinemia (H), or non‐Hodgkin's lymphoma (NHL). In addition, hyper IgM, red cell aplasia, necrotizing lymphoid vasculitis (NLV), and aplastic anemia occur rarely. The recent use of DNA restriction fragment length polymorphism (RFLP) probes in linkage with the XLP gene now permit detection of affected males prior to primary EBV infection. We have measured immunoglobulin class and subclass levels in sera from EBV‐negative males who were either positive or negative for the XLP genotype by RFLP analysis. Elevated IgA or IgM and/or variable deficiency of IgG, IgG₁, and IgG₃ occurred in the sera of 13/13 RFLP‐positive, EBV‐negative males. No consistent abnormalities were noted in 14 RFLP‐negative, EBV‐negative males. We conclude that the immune defect in XLP is not solely EBV‐specific, although EBV is responsible for most of the morbidity and all of the mortality. Further, serial measurement of Ig levels may provide information regarding status of EBV‐negative males at risk where RFLP analysis is uninformative or in families where sporadic cases of fatal IM, acquired hypogammaglobulinemia or NHL have occurred, but wherein the genotype of XLP cannot be documented.