Individuals with X-linked lymphoproliferative disease are susceptible to severe Epstein-Barr virus (EBV) infections that are often fatal. Mutations in signaling lymphocytic activation molecule-associated protein (SAP) are associated with this illness. We describe a patient with a novel serine-to-proline mutation at aa 57 in SAP and compare the location of the altered amino acid with all known missense mutations in the SAP-encoding SH2D1A gene, including those of 4 additional individuals whose cases have not been described elsewhere. The patient's genetic condition was discovered only after he exhibited an abnormal host response to primary EBV infection that resulted in hemophagocytic lymphohistiocytosis syndrome, which was complicated by marrow aplasia with terminal disseminated aspergillosis.