<h3>Introduction</h3> SLAM–associating protein deficiency (SAP deficiency), also known as X-linked lymphoproliferative disease (XLPD Type 1), is an inborn error of immunity caused by a hemizygous variant in the <i>SH2D1A</i> gene, which encodes this protein. SAP is essential for immunity to Epstein Barr Virus (EBV) with deficiency leading to fulminant EBV infection, hemophagocytic lymphohistiocytosis (HLH) and B-cell lymphomas. Herein, we describe a child who presented with recurrent B cell lymphoma and was determined to have SAP deficiency. <h3>Case Description</h3> The patient is a 7 yo previously healthy male who presented with throat pain, enlarged tonsils and cervical lymphadenopathy. He failed multiple courses of antibiotics, and tonsillar incision and drainage was ineffective. Tonsillar biopsy showed mature large B-cell lymphoma, stage II. He responded to chemotherapy, but after 3 months off therapy had relapse of disease with a tonsillar mass, confirmed by biopsy. On further work-up, testing showing very low SAP expression in lymphocytes: %SAP expressed in CD8+ Tcells 1% (ref range >62%), CD8- T cells 0% (ref range >64%), CD56 NK cells 0% (ref range >55%). XIAP expression in lymphocytes was normal. This was consistent with XLPD Type 1 or SAP deficiency. <h3>Conclusion</h3> Patients with SAP may present in infancy or in childhood with clinical presentation derived by an increased susceptibility to EBV. However, the phenotype is variable making recognition challenging without fulminant EBV infection, HLH or hypogammaglobulinemia. Immunologists should have a high suspicion for SAP deficiency in patients with recurrent lymphomas as diagnosis changes management with hematopoietic stem cell transplant leading to cure.