Objective To explore the relationship between Adult-onset Still's disease (AOSD) and macrophage activation syndrome (MAS). Methods A total of 78 patients with AOSD who had completed medical information were included in this study. Eleven patients who were diagnosed as rheumatic disease associated hemophagocytic syndrome among 26 patients who had hemophagocytic syndrome with histological evidence consisted of the MAS group. Clinical and laboratory data were analyzed in 78 patients with AOSD and 11 patients with MAS. Results Among 78 cases of AOSD, 9 patients (12%) could be diagnosed as MAS but didn't have hemophagocytic histological evidence. In the 11 MAS cases with hemophagocytic phenomenon, 6 patients fulfilled the diagnostic criteria of AOSD, 2 cases with panniculitis, 1 case with SLE, 1 case of dermatomyositis and 1 case of systemic vasculitis. Logistic analysis showed that splenomegaly (OR =2.13, 95%CI=1.11-3.42), leukopenia (OR=3.57, 95%CI=2.30～4.86), anaemia (OR=0.85, 95%CI=1.03～2.76), thrombocytopenia (OR=2.98, 95%CI=1.17-4.30) and hypertriglyceridemia (OR=1.66, 95%CI=1.02～2.74) were associated with development of MAS in AOSD. Conclusion The development of MAS in AOSD patient is frequent and hemophagocytic histological evidence could be found in severe cases. When splenomegaly and hypocytomsis present in AOSD patients, bone marrow examination should be done and the level of triglyceride and fibrinogen and activity of NK cells should be measured for early diagnosis.