Objective To assess the clinical features, laboratory findings, and response to therapy according to disease course, and analyse the predictive factors for unfavourable outcomes in patients with adult-onset Still's disease (AOSD). Methods We retrospectively reviewed the medical records of 82 patients from January 1992 to December 2010 at a single tertiary hospital. Thirty-three had monocyclic disease, 33 experienced at least one relapse, and 14 had chronic disease. Patients were divided into those with favourable (monocyclic, n=33) and unfavourable (polycyclic or chronic and death, n=49) outcomes. Results The major clinical features were high spiking fever (96.3%), polyarthralgia (85.4%), skin rash (80.5%), myalgia (70.7%), and sore throat (68.3%). Analysis of prognostic factors for the 2 groups showed that polyarthralgia, elevated erythrocyte sedimentation rate, high serum lactate dehydrogenase, and low dose of initial glucocorticoids were related with unfavourable outcomes. An insufficient starting dosage of prednisolone or its equivalent (<30 mg/day) was the most significant predictive factor (OR 6.476, p=0.007) for chronic and relapsing disease, markedly decreasing response rates. Conclusion Although AOSD is a benign disease, relapses are common and a chronic disease requires immunosuppressive therapy, that these unfavourable patients show significantly longer time from initiation of treatment to remission. Hence, it is important to control disease activity at the start of treatment with sufficient glucocorticoids.