BACKGROUND Adult-onset still disease (AOSD) and hemophagocytic syndrome (HPS) are two inflammatory diseases with very similar clinical manifestations. HPS is one of the most serious complications of AOSD and its risk of death is very high. It is difficult to identify HPS early in patients with AOSD, but early identification and proper treatment directly affects the prognosis. CASE SUMMARY A 39-year-old male showed a high spiking fever and myalgia. Laboratory data revealed elevated white blood cell, serum ferritin, and neutrophil percentage. However, his fever failed to relieve after a clear diagnosis of AOSD caused by pulmonary infection and treatment by antibiotics and corticosteroids; further laboratory data showed elevated serum ferritin, C-reactive protein, erythrocyte sedimentation rate and triglyceride, as well as liver abnormalities. Bone marrow smear showed hemophagocytosis. Secondary HPS was definitely diagnosed. The high fever disappeared and the laboratory findings returned to normal values after treatment by high-dose intravenous methylprednisolone and methotrexate. CONCLUSION For AOSD patients with high suspicion of HPS, active examination needs to be considered for early diagnosis, and timely using of adequate amount of corticosteroids is the key to reducing risk of HPS death.