BACKGROUND Adult-onset Still's disease (AOSD) is a rare multi-systemic inflammatory disorder of unknown etiology characterized by spiking fever, characteristic rash, and arthritis. It often associates with high serum ferritin levels. CASE REPORT An 88-year-old woman had fever of over 39°C without response to extended-spectrum antibiotics for 6 days. She had non-specific erythema with infiltration on her trunk. She had leukocytosis with neutrophilia of 80%, mild hepatic dysfunction, normal level of rheumatoid factor and antinuclear antibody, thrombocytopenia, elevated d-dimer and soluble interleukin2 receptor, extremely high serum ferritin (78 662 ng/mL), and splenomegaly. Although she had no arthritis or specific erythema, we made the diagnosis of AOSD according to Yamaguchi's criteria with disseminated intravascular coagulation (DIC) and hemophagocytic syndrome (HPS) after ruling out infections, malignancies, or other connective tissue diseases. Twelve percent of AOSD patients have HPS. The mean serum ferritin of AOSD with HPS was reported at 18 179 ng/mL, which supported the diagnosis of AOSD because only a few other diseases could show such extremely high serum ferritin. Although she was treated with prednisolone (30 mg/day), her condition deteriorated and her left pleural effusion increased. Therefore, methylprednisolone 500 mg/day for 3 days was started followed by prednisolone 30 mg/day and immunosuppressive agent (Cyclosporine 50 mg/day), which improved her general condition, elevated C-reactive protein levels, and extremely high serum ferritin levels. CONCLUSIONS We report the case of an elderly patient with severe AOSD, who developed HPS and DIC, whose extremely high serum ferritin level was useful in diagnosis.