Hemophagocytic syndrome consists of primary and secondary HLH. Efficacy of therapeutic measures and prognosis depend on degree of hypercytokinemia-associated organ failure at disease onset and underlying disorders. The underlying diseases related to hemophagocytosis and informative markers useful for differential diagnoses to select the most effective treatment are discussed. Differential diagnosis is difficult between confirmed FEL and familiality-unknown infantile VAHS (or sporadic FEL cases) in primary HLH and also among IAHS, benign EB-VAHS and EBV-related LAHS in secondary HLH. For primary HLH, assay of NK activity and for secondary HLH, studies on the serum cytokine pattern, EBV genomes and clonality determination might prove useful.