We report the case of a 31-year-old woman who developed adult-onset Still's disease (AOSD) with a high level of serum interleukin (IL)-18. Although treated with high dose steroids, she suffered repeated remissions and her condition deteriorated. After we administered oral cyclosporine A (CsA), 200 mg/d, monitoring C2 and trough levels, her symptoms improved significantly. We decreased the dose of methylprednisolone slowly without noting a relapse. The use of CsA accompanied by C2 and trough level monitoring should be considered for refractory AOSD patients with high levels of serum IL-18.