Still's disease is a clinical entity of unknown origin, which can appear before 15 years of age (juvenile onset Still's disease) or later (adult onset Still's disease). There are few reported data about the long term prognosis of Still's disease and no study compares the long term evolution of adult onset and juvenile onset Still's disease. Eighteen patients fulfilling the American Rheumatism Association criteria for Still's disease were followed up for more than 10 years. Ten (group 1) had juvenile onset Still's disease and eight (group 2) adult onset Still's disease. A comparison of the groups showed no significant differences in the initial systemic manifestations of Still's disease, or in the joint lesions. Both groups had severe sequelae, which appeared between six and 10 years after the initial flare up of Still's disease. Nine patients had articular damage and nine had only arthritis without apparent x ray abnormalities. Nine patients had bilateral hip destruction in less than four years. Of these nine, seven required 13 total hip replacements before the age of 45. In the whole group of 18 patients bilateral involvement of the following joints was also seen: carpus (seven patients), knee (four), tarsus (four), ankle (three); three patients had ankylosis of the cervical spine. The occurrence of amyloidosis (three cases, two deaths) was restricted to group 2. This was the only difference between the groups, as the treatments were identical. It is concluded that the articular prognosis of Still's disease is poor, be it adult onset or juvenile onset, with severe joint destruction in half of the patients.