A 52-year-old woman was admitted to our hospital because of a skin rash, high fever and myalgia. She had been diagnosed ten years ago by a dermatologist as having MCTD (mixed connective tissue disease). At the time of admission a diagnosis of active SLE was made by fulfilling four of the 1982 ARA criteria together with increasing levels of anti-DNA antibody and low levels of complements. Prednisolone (PSL) given orally in an initial dosage of 60 mg/day was effective during the first 6 weeks. Then a high fever, skin rash and pancytopenia appeared without active findings of SLE. Infection caused by bacteria, fungus or virus was suspected, but no infectious agent was present in cultures derived from blood or other sources. Antimicrobic drugs used were not effective at all. The clinical picture was suggestive of a drug allergy, but no causative drug was found. A diagnosis of hemophagocytic syndrome (HPS) was made because of the increased number of unusual hemophagocytic cells in the bone marrow. High levels of serum ferritin and neopterin, which are known to reflect macrophage activation, supported the diagnosis of HPS. HPS is characterized by activated phagocytosis presumably induced by hypersecretion of cytokines. Malignant lymphoma and infection are the two representative diseases which may cause HPS. Recently, an acute lupus HPS was reported in patients with active SLE. Here we reported a case of reactive HPS observed in a patient with SLE who had been receiving high dose PSL. Symptoms and findings of the patient gradually disappeared in several weeks after rapid reduction of the PSL dose.(ABSTRACT TRUNCATED AT 250 WORDS)