An 11-year-old Turkish girl from a non-consanguineous family was suffering from joint pain, fever, hepatosplenomegaly, and respiratory insufficiency. Laboratory abnormalities were thrombocytopenia, elevated levels of serum transaminases, lactate dehydrogenase, and C-reactive protein (up to 193 mg / l), a hyperferritinaemia of 8030 ng/ml, and an increased sCD25. The tentative diagnosis of macrophage activation syndrome (MAS) was confirmed by the detection of a histiocytosis with haemophagocytosis in the bone marrow. Treatment with dexamethasone, cyclosporine A, and VP16 was successful. However, the diagnosis of MAS on the background of a systemic juvenile idiopathic arthritis was questionable because of recurrent, spontaneously remitting fever phases of 5 to 7 days duration without an obvious infectious aetiology. A positive family history of febrile episodes in three consecutive generations raised the suspicion of a dominantly inherited disease. Genetic studies revealed a likely pathogenetically relevant E56D/p.Glu85Asp mutation in exon 3 of the TNFRSF1A gene. Alterations of the MEFV gene, in contrast, were not found. To our knowledge, this is the first case of a macrophage activation syndrome as the initial manifestation of TRAPS. Similar case reports in patients with the far more common familial Mediterranean fever (FMF) have been published already.