To date, the treatment of adult-onset Still's disease (AOSD) has been largely empirical; therefore, this study was conducted to investigate the response to therapy and prognostic factors of AOSD. Fifty-four Korean patients with AOSD were enrolled based on Yamaguchi's criteria. We retrospectively analyzed the treatments and prognosis. Thirty-nine patients (72.2%) were female, and the average age at disease onset was 37.3 years. Twenty-nine patients had a monocyclic disease (53.7%), five had a polycyclic (9.3%) and fifteen had a chronic articular disease (27.7%) and five died (9.3%). The elevated ESR and corticosteroids refractoriness were associated with poor prognosis (P = 0.023 and P = 0.009, respectively). The patients that died were older than those survived (49.2 ± 11.8 vs. 42.2 ± 14 year old, P = 0.024). Forty-two patients were treated with non-steroidal anti-inflammatory drugs; however, they also needed corticosteroids and intravenous immunoglobulin (IVIG). Among 50 patients treated with high-dose corticosteroids, 21 patients (42%) were resistant to corticosteroids and treated with IVIG or anti-tumor necrosis factor (TNF) agents. Of the 23 patients medicated with IVIG, the prognosis was better in IVIG-responsive patients, indicating a therapeutic effect. Methotrexate was the most commonly used disease modifying anti-rheumatic drugs (27 patients, 50%), and the corticosteroid requirements were lower in the methotrexate-responsive patients. Approximately half of AOSD patients had a poor prognosis and were corticosteroids resistance. An elevated ESR and non-response to corticosteroids were associated with poor prognosis. Patients who died were older than those survived.