An 8-year-old maternally related relative of three boys who had developed agammaglobulinemia associated with Epstein-Barr virus (EBV)-induced infectious mononucleosis was studied for X-linked lymphoproliferative syndrome (XLP) in 1979. At that time, he demonstrated no striking immunologic aberrations and was seronegative for EBV. Subsequently, immunologic abnormalities including failure to switch from IgM to IgG antibody synthesis after secondary immunization with bacteriophage phi X174 were detected. In 1983, he experienced episodic intracerebral hemorrhages, with the second being fatal. At autopsy, necrotizing vasculitis and aneurysms involving arteries of the central nervous system were observed. Studies of blood obtained immediately before and after death failed to show antibodies to EBV. However, EBV genome was demonstrated in tissues obtained at autopsy by DNA hybridization studies. Fatal lymphoid vasculitis in this patient is unique among boys with XLP in the registry. These findings probably extend the phenotypic expressions of XLP.