Background. The reactive haemophagocytic syndrome (RHS) is a little‐known life‐threatening complication of rheumatic diseases in children. It reflects the extreme vulnerability of these patients, especially those with systemic‐onset juvenile chronic arthritis (JCA). This immunohaematological process may be triggered by events such as herpes virus infection and non‐steroidal anti‐inflammatory drug therapy. Treatment has not been standardized. Methods. We characterized this unusual disorder and determined its incidence by carrying out a retrospective study of patients identified over a 10‐yr period in French paediatric units. Results. Twenty‐four cases (nine males, 15 females) were studied. Eighteen had typical systemic‐onset JCA, two had polyarthritis, two had lupus and two had unclassifiable disorders. Clinical features at diagnosis included high spiking fever (24 patients), enlargement of the liver and spleen (14), haemorrhagic diathesis (six), pulmonary involvement (12) and neurological abnormalities (coma or seizures) (12). RHS was the first manifestation of systemic disease in three cases. Admission to intensive care was required in ten cases. Hypofibrinogenaemia, elevated liver enzymes and hypertriglyceridaemia were found consistently. Phagocytic histiocytes were found in 14 of 17 bone marrow smears. RHS was presumed to have been precipitated by infection in 11 cases (four Epstein–Barr virus, three varicella‐zoster virus, one parvovirus B19, one Coxsackie virus, one Salmonella, one Pneumocystis carinii) and by the introduction of medication in three cases (Salazopyrin plus methotrexate; morniflumate; aspirin). Macrophage activation was indicated by high levels of monokines in the serum of two patients. Twenty patients had only one episode, three had an early relapse and one patient had two relapses. The treatment regimen was tailored to each child as the clinical course was variable. There was no response to intravenous immunoglobulins, which were used in four cases. Intravenous steroids at doses ranging from conventional to pulse methylprednisolone induced remission in 15 of 21 episodes when used alone as the first‐line treatment. Cyclosporin A was consistently and rapidly effective, both when used as second‐line therapy in all seven of the episodes in which steroids failed and in all five patients who received it as their first‐line treatment. This supports a central role of T lymphocytes in the haemophagocytic syndrome. Two patients died. One patient with lupus died of congestive fulminant heart failure after 4 days, despite treatment with intravenous steroids and immunoglobulins, and one patient with systemic‐onset JCA died from multiorgan failure despite aggressive therapy with pulsed steroids and etoposide. Conclusions. RHS may be a more common complication of systemic disease in childhood than previously thought. This life‐threatening complication should be diagnosed promptly, as it calls for the immediate withdrawal of potentially triggering medications, anti‐infective therapy when relevant, and urgent immunosuppressive treatment, measures that are very often effective. Cyclosporin A may be the drug of choice.