Hemophagocytic lymphohistiocytosis is a potentially fatal disease characterized by excessive macrophage and lymphocyte activity. Patients can be affected following immune activation after an oncologic, autoimmune or infectious trigger. An associated gene mutation may be found which impairs cytolytic lymphocyte function. We describe a pediatric case of hemophagocytic lymphohistiocytosis with a novel mutation of MUNC 13-4 whose diagnosis was confounded by concurrent immunosuppression. Clinical reassessment for hemophagocytic lymphohistiocytosis is necessary in persistently febrile patients with laboratory derangements in the setting of immunosuppressive agent exposure.