A 7-year-old female suddenly exhibited high fever and convulsions, and entered a semi-coma. She also had thrombocytopenia, elevated aminotransferase, prolonged prothrombin time and activated partial thromboplastin time, and hemophagocytes in the bone marrow. The brain magnetic resonance imaging revealed multiple low-intensity areas on the T₁-weighted images, and high-intensity areas on the T₂-weighted images bilaterally in the thalamus, the dorsal part of the pons, and the cerebellar white matter. The patient was diagnosed as having both acute necrotizing encephalopathy and hemophagocytic syndrome. Serum and cerebrospinal fluid interleukin-6 and tumor necrosis factor-alpha were elevated to the same high levels (serum:cerebrospinal fluid interleukin-6, 103:101 pg/mL; tumor necrosis factor-alpha 753:753 pg/mL). The clinical symptoms and the magnetic resonance imaging findings improved immediately after the administration of dexamethasone. These results suggest that the hypercytokinemia and the hyperpermeability of both the blood-brain barrier and the capillary walls of the central nervous system might be essential in the pathogenesis of acute necrotizing encephalopathy, and that early steroid therapy might be effective in these conditions.