We conducted a retrospective observational study to describe a cohort and identify the prognostic factors in adult-onset Still disease (AOSD). Patients enrolled in this retrospective chart review fulfilled either Yamaguchi or Fautrel criteria. Candidate variables were analyzed with logistic unadjusted and adjusted regression models. Fifty-seven patients were seen in the internal medicine (75%) and rheumatology (25%) departments over a mean period of 8.4 years. The median time to diagnosis was 4 months. The course of AOSD was monocyclic in 17 patients, polycyclic in 25, and chronic in 15. The assessment of glycosylated ferritin (GF) in 37 patients was correlated with early diagnosis. Nine F-18-fluorodeoxyglucose positron emission tomography ((18)FDG-PET) scans identified the lymph nodes and glands as the main sites of hypermetabolism. Complications were frequent (n = 19), including reactive hemophagocytic syndrome (n = 8). None of the 3 deaths could be attributed to AOSD. Corticosteroid dependence, as predicted by a low GF level, occurred in 23 patients (45%). A quarter of the patients received tumor necrosis factor-alpha blockers or anakinra with good tolerance. Fever >39.5 degrees C was predictive of monocyclic AOSD, while arthritis and thrombocytopenia were associated with chronic and complicated AOSD, respectively. The youngest patients had the highest risks of resistance to first-line treatments. AOSD remains difficult to diagnose. Mortality is low despite frequent complications. GF and (18)FDG-PET scans were of value in the diagnostic approach. The condition in highly symptomatic patients evolved to systemic AOSD, whereas more progressive patterns with arthritis predicted chronic AOSD.