Primary lymphoma of the central nervous system in two children with acquired immune deficiency syndrome. Postmortem examination disclosed central nervous system non-Hodgkin's lymphoma in two children who died of acquired immune deficiency syndrome (AIDS) at 6 and 14 months of age, respectively. Systemic signs of lymphoma were not present. The B-cell origin and clonality of the neoplastic cells were established by immunohistochemistry in one case and by molecular analysis of immunoglobulin gene rearrangement in the other. Moreover, in the latter case the neoplastic cells were characterized by the presence of a single episomal EBV genome. According to these data, the monoclonal B-cell proliferation occurred after EBV infection, thus suggesting a possible pathogenetic role of EBV in the early stages of lymphomagenesis. 