We report the occurrence of X-linked lymphoproliferative disease (XLP) in two brothers in a Malaysian family. In this disorder, a primary Epstein-Barr virus (EBV) infection is followed by an abnormal proliferation of transformed B-cells that cannot be controlled by suppressor T-cells, leading to the development of deranged immune function. This results in fatal infectious mononucleosis, acquired hypogammaglobulinaemia, virus-infected haemophagocytic syndrome and non-Hodgkin's lymphoma. The diagnosis should be considered when there is a family history of any male having a fulminant course of infectious mononucleosis, an otherwise benign disease. Early diagnosis is important as bone marrow transplantation is the only curative option in this disorder.