Syndrome of hemophagocytic lymphohistiocytosis (HLH) is characterized by uncontrolled proliferation of T lymphocytes and macrophages. HLH develops in patients with primary or secondary immunodeficiency. Overproduction of cytokines plays a major role in the tissue damage and in the development of typical clinical and laboratory features - persistent fever, hepatosplenomegaly, peripheral blood cytopenia, hypertriglyceridemia, hypofibrinogenemia and bonemarrow hemophagocytosis.Two forms ofHLHexist: primaryHLH - a heterogenous group of rare genetic disorders and a secondary form, associated with systemic infections, malignancies or systemic autoimmune diseases. HLH is a life-threatening acute illness frequently progressing into the multiorgan failure and death. Establishment of diagnosis and the appropriate choice of therapy are often difficult. Both forms ofHLH respond well to the immunosuppressive therapy. Stem cell transplantation offers the only curative treatment strategy for children with the primary HLH. Diagnostic and therapeutic dilemma is demonstrated on case-reports of patients suffering from familial hemophagocytic lymhohistiocytosis, Chediak-Higashi syndrome, fatal infectious mononucleosis, visceral leishmaniasis, large cell anaplastic lymphoma, secondary malignant histiocytosis and macrophage activation syndrome associated with juvenile idiopathic arthritis. Awarness of differential diagnosis of HLH, application of modern diagnostic methods and an early initiation of the efficient treatment improve the prognosis of these unfavourable disorders.

        Key words: hemophagocytic lymphohistiocytosis, immunodeficiency, infection, macrophage activation syndrome, stem-cell transplantation