X-linked lymphoproliferative disease is a rare T and NK cell immune deficiency which most frequently presents as fulminant infectious mononucleosis following infection with the Epstein-Barr virus (EBV). We report the case of a 4-year-old boy from a Spanish family presenting with severe infectious mononucleosis. In the course of the disease he developed hepatic failure, pancytopenia and neurologic impairment, leading to death after less than 2 months. The results of bone marrow biopsy and autopsy indicated a histological diagnosis of both high-grade B-cell lymphoma and virus-associated haemophagocytic syndrome, thereby confirming the simultaneous presence of two different manifestations of X-linked lymphoproliferative disease (XLP) in this patient. The family history revealed four close male relatives dying under similar circumstances, one of whom died following a vaccination against measles. Molecular genetic studies identified a novel mutation in the SH2D1A gene in several members of the family, establishing the diagnosis of XLP. Fatal EBV infection in male infants is highly indicative of XLP. Virus-associated haemophagocytic syndrome and B-cell lymphoma can occur concomitantly and may be difficult to distinguish due to their similar histological pictures.