Hemophagocytic lymphohistiocytosis (HLH) is a rare histiocytic reactive process due to mutations in the perforin, MUNC13-4 or syntaxin 11 genes, or secondary to malignancy, infection or autoimmune disorder. HLH as a preceding diagnosis to leukemia is rare. We report two cases with progression to acute leukemia, one heterozygous for MUNC13-4 and the other with reduced natural killer (NK) cell function and perforin expression. These defects may predispose to a secondary HLH-like presentation of pre-clinical leukemia or confer increased susceptibility to malignancy. HLH patients with genetic mutations or NK cell function abnormalities need monitoring for future malignancy even if the HLH resolves.