To examine the prevalence and characteristics of patients with reactive haemophagocytic syndrome (RHS) complicating adult-onset Still's disease (AOSD). Of 50 patients with AOSD fulfilling Yamaguchi and Fautrel criteria followed in our department, clinical and laboratory data, course and treatment of six patients with histologically proven RHS and without any obvious cause other than AOSD were retrospectively recorded. RHS led to AOSD in two cases, whereas it appeared after a mean duration of 3.5 years from onset of AOSD in the other cases. The main symptoms were fever (n = 6), polyarthralgias or myalgias (n = 4), lymphadenopathy or splenomegaly (n = 3), pharyngitis (n = 3), rash (n = 3), pleuritis (n = 3), hepatomegaly (n = 1), normal or low leucocyte count (n = 4), anaemia (n = 6), lymphocytopenia (n = 6), thrombocytopenia (n = 4), hyperbasophilic lymphocytes (n = 2), abnormal liver function tests (n = 6) and increased serum triglyceride level (n = 6). Serum ferritin concentration was constantly increased (>10,000 microg/l in five cases, with <5-35% in glycosylated form). Two patients presented with coagulopathy. Treatment comprised corticosteroids (n = 4) and intravenous immunoglobulins (n = 3), whereas prednisone was unchanged in one case. One death due to pneumonia occurred 15 days after RHS. With a follow-up ranging from 2 to 7.5 years, the other patients were in remission with prednisone plus etanercept (n = 1), prednisone plus methotrexate (n = 1), low-dose prednisone (n = 2) or without treatment (n = 1). RHS is not uncommon in AOSD. It should be evoked in a patient with AOSD in the absence of hyperleucocytosis, thrombocytopenia, lymphopenia and coagulopathy, or in the presence of high serum ferritin and triglyceride levels.