Familial haemophagocytic lymphohistiocytosis (FHL) is a rare, autosomal recessive disease of infancy and early childhood clinically characterized by fever, hepatosplenomegaly, lymphadenopathy, rash, neurological symptoms and icterus. Common laboratory findings include cytopenia, elevated liver enzymes, hyperbiliriubinaemia, hypofibrinogenaemia and hypertriglyceridaemia. The natural killer cell function is frequently decreased or absent. A diffuse lymphohistiocytic infiltration is seen in the reticuloendothelial system, often with haemophagocytosis. Molecular diagnosis is available in a minority of FHL families. Without adequate treatment and bone-marrow transplantation, the disease is fatal. A 6-wk-old child with FHL is presented. Shortly before the clinical onset of the disease, blood testing and bone-marrow examination had been carried out. All results were considered normal at that time. Conclusion: Blood tests and bone-marrow examination may be normal shortly before the clinical presentation and therefore do not exclude the diagnosis of FHL. There is a need for extended molecular diagnostic possibilities.