Hemophagocytic Lymphohistiocytosis (HLH) is a rare syndrome characterized by ineffective T-cell and NK response. We report the clinical course of a patient with relapsed CLL who developed acute symptoms soon after starting ibrutinib. Hyperpyrexia, splenomegaly, hyperferritinemia, hypertriglyceridemia, cytopenias, and a typical cytokine pattern, i.e. high interleukin (IL)-6, IL10 and IL18, were consistent with a diagnosis of HLH. Coexistent Epstein Barr virus reactivation was documented. Ibrutinib-induced impairment of NK degranulation, associated with EBV reactivation and CLL-related immunodeficiency may have contributed to the development of HLH in our patient.