Hemophagocytic lymphohistiocytosis is an immune dysregulatory syndrome that is associated with alteration in the immune response activation and inhibition balance. There are two basic forms of the syndrome: primary (genetic or familial) determined by genes mutations involved in immune cell interactions, and secondary or sporadic developing as a result of an infectious process. The exact genetic background of the secondary form is still unknown. These forms are characterized by same combination of specific hyperinflammatory reactions and clinical signs and symptoms. Discrimination between primary and secondary forms is often challenging due to the rarity of the pathology, a wide spectrum of clinical signs, and limited availability of specific tests. Etiopathogenetic treatment of the primary form is an urgent allogeneic hematopoietic stem cell transplantation, otherwise a fatal outcome is inevitable. Meanwhile, the approach to the secondary form depends on the clinical manifestation and the type of the infectious trigger. To rescue the patient, a timely diagnosis is crucial for prompt administration of appropriate treatment. Treatment of hemophagocytic lymphohistiocytosis is complicated by the high incidence of treatment-related mortality and the propensity to relapse. Keywords: hemophagocytis lymphohistiocytosis, immunodeficiency, diagnostic criteria.