We retrospectively studied the incidence of familial hemophagocytic lymphohistiocytosis (FHL) in children during the 16-year period 1971-86. First, all departments of pediatrics, pathology, and infectious diseases were enquired for children with FHL or disorders resembling FHL. Secondly, the causes of death of all children who died during the study period in Sweden (n = 19,542) were also investigated. Files and histological specimens were further studied in selected children. By using a set of inclusion/exclusion criteria, we found 32 children with FHL. The incidence was 1.2/1,000,000 children per year. One child per 50,000 live borns developed FHL during this period. The sex ratio was close to 1:1. Prominent early clinical signs were fever (91%), splenomegaly (84%), hepatomegaly (90%), rash (43%), and lymph node enlargement (42%). Neurological symptoms, which developed in 47%, could totally dominate the clinical picture and develop prior to other symptoms and signs. Common laboratory findings were pancytopenia, hypertriglyceridemia, hypofibrinogenemia, elevated serum transaminases, hyperbilirubinemia, hyponatremia, hypoalbuminemia, and a moderate spinal fluid pleocytosis. Chest X-ray often revealed mostly discrete pulmonary infiltrates. FHL is an underdiagnosed disease and in only 11/32 children was diagnosis made during their lifetime. It is important to be aware of the disorder as potential therapy now exists.