A 65-year-old woman with adult Still's disease developed adult respiratory distress syndrome (ARDS), a fatal pulmonary complication. Intravenous administration of cyclophosphamide, 500 mg/d for three days, was much more effective than high doses of corticosteroids in the patient. Interestingly, hypersensitivity to flavoxate hydrochloride seemed to be a precipitating factor, but not a cause, for both a series of characteristic manifestations of adult Still's disease and development of ARDS in our patient. The association of ARDS with adult Still's disease has not yet been reported. Physicians should be aware of this fatal complication in adult Still's disease, especially in the presence of drug hypersensitivities.