Haemophagocytic lymphohistiocytosis (HLH) is a life threatening inflammatory syndrome, which presents a highly stimulated but ineffective immune response with severe hypercytokinaemia. HLH, primary or secondary, is characterised by prolonged fever and hepatosplenomegaly associated with pancytopenia, hypertriglyceridaemia and hypofibrinogenaemia. However, the hallmark of HLH is impaired or absent function of natural killer cells and cytotoxic T lymphocytes. HLH presents major diagnostic difficulties, since it may have an incomplete and/or late onset and with many conditions leading to the same clinical picture. When untreated, it is fatal in all primary cases and in a high percentage of acquired cases. Awareness of the clinical picture and diagnostic criteria is thus important to start life saving treatment. We describe two cases of primary HLH, with significant differences in their clinical presentation and evolution.