Adult onset Still's disease was first reported by Bywaters in 1971. It is a systemic inflammatory disorder of unknown etiology, characterized by spiking fever, macular rash and polyarthritis. Although the prognosis is generally good, severe cases have been published. They include those with disseminated intravascular coagulation (DIC), hemophagocytosis, amyloidosis and respiratory failure. Among them, DIC is not uncommon. Prednisolone in a dose of 20-60 mg/day is required when patients fail to respond to nonsteroidal anti-inflammatory drugs (NSAIDs) or when they are accompanied by complications including pleuritis, pericarditis, liver dysfunction, severe arthritis and DIC. Recently, disease-modifying antirheumatic drugs (DMARDs) and immunosuppressive agents including cyclophosphamide and methotrexate have been shown to be effective for alleviating refractory cases and chronic arthritis.