Reactive hemophagocytic syndrome (RHS) is a rare, life-threatening, and little-known complication of rheumatic diseases. This disorder is characterized by fever, pancytopenia, liver failure, coagulopathy, and neurologic symptoms. RHS may develop in patents who have lymphoma, organ transplantation, serious infection, and rheumatic diseases, most notably systemic lupus erythematosus and adult-onset Still disease (AOSD). Observations of specific cases of RHS in AOSD remain rare, and the significance of this syndrome during the course of AOSD remains unknown. We retrospectively studied 16 episodes of AOSD-associated RHS in 8 patients. To determine whether RHS is associated with a particular phenotype of AOSD, we conducted a case-control study from the cohort of AOSD patients seen during the same period. The estimated frequency of RHS in AOSD patients from our cohort was 15.3% (8/52). The median age at RHS diagnosis was 44.5 years. We collected clinical and laboratory data. RHS was the first manifestation of AOSD in 7 cases. The main symptoms were fever (n = 8), salmon rash (n = 6), arthralgia (n = 7), lymphadenopathy (n = 6), and shock (n = 4). Serum ferritin concentration was consistently elevated (>1000 microg/L in 8 cases), and the level of glycosylated ferritin was low in all cases (<5% in 7 cases, 15% in 1 case). Six patients presented with coagulopathy; hypertriglyceridemia was found in 6 cases. Admission to the intensive care unit was required in 4 cases. Treatment included corticosteroids (n = 8) and intravenous immunoglobulin (n = 6), cyclophosphamide in 2 cases, infliximab in the same 2 cases, and cyclosporine in 1 case. With a follow-up ranging from 2 to 15 years, the patients were in remission with prednisone plus methotrexate (n = 4), prednisone plus infliximab (n = 2), and low-dose prednisone alone (n = 2). We compared the 8 patients included in this study with 44 control patients with AOSD without RHS. Low haptoglobin levels, very high ferritin levels (>10,000 microg/L), and a normal or low neutrophil count seem to be predictive factors of the occurrence of RHS in AOSD.