The X-linked lymphoproliferative syndrome (XLP), also known as Duncan's disease, is an X-linked recessive disorder that is characterized by the inability of affected individuals to mount a sufficient immune response to Epstein-Barr virus (EBV). After EBV primary infection, male family members suffer from severe infectious mononucleosis (IM), aplastic anemia, hypogammaglobulinemia, and a spectrum of lymphoproliferative diseases. Autosomal mode of inheritance with similar symptoms as in XLP has been reported. We have studied two families with EBV-associated syndromes and an X-linked and an autosomal mode of inheritance, respectively. Affected family members presented with severe IM, hemophagocytosis, aplastic anemia, acquired hypogammaglobulinemia, and B-cell lymphoproliferative diseases.