OBJECTIVES: To evaluate the efficacy of cyclosporine A in the treatment of macrophage activation syndrome (MAS) occurring in children with juvenile arthritis. STUDY DESIGN: MAS developed in two boys and three girls with systemic juvenile arthritis (four) and polyarticular juvenile arthritis (one). In three children whose condition was life-threatening, increased parenteral administration of corticosteroids failed to improve their condition; therefore cyclosporine A (2 to 5 mg/kg per day) was added. In two other patients with less severe clinical manifestations, cyclosporine A alone (2 to 8 mg/kg per day) was given. RESULTS: After the introduction of cyclosporine A, rapid improvement was obtained in all patients and apyrexia occurred within 24 to 48 hours. The biologic abnormalities disappeared more slowly (up to 5 weeks for liver enzymes). CONCLUSIONS: These observations underline the usefulness of cyclosporine A in this complication. The use of this drug may circumvent the need for increased doses of corticosteroids in some patients. The mechanism of action of cyclosporine A remains speculative, but these results indicate indirectly that T-helper lymphocytes may play a role in the pathogenesis of MAS.