Introduction : Acute necrotizing encephalopathy (ANE) is rare but distinctive type of acute encephalopathy. Clinical course and the prognosis of ANE are diverse from mild to severe forms. ANE is characterized by acute encephalopathy with bilateral, symmetrical lesions in thalami, brainstem predominantly usually after viral infection. ANE should be ruled out other causes of encephalopathies including Reye syndrome, Leigh encephalopathy, and Wernicke encephalopathy. We report a 15-month-old boy with acute encephalopathy with reversible, bilateral thalamic lesions as a mild-form variant of ANE. Case reports : A 15-month-old boy was admitted because of a generalized tonic-clonic convulsion lasting for 2 hours. Two days before admission, he had developed a dry cough, mild rhinorrhea. He had taken antibiotics and antipyretics before admission. Family and past histories were not contributory. Physical examinations were unremarkable except for sedated mentality postictally. Body temperature was 37.9’℃. Labaratory findings were as follows: serum aspartate aminotransferase 30U/L, alanine aminotransferse 13U/L, ammonia 84mmol/L, lactate 0.8mmol/L, glucose 183mg/dL. The cerebrospinal fluid showed 2 WBC/μL with protein level of 15.4 mg/dL. Influenza virus was negative from nasopharyngeal secretion. Brain computed tomography scan showed no abnormal findings. Intravenous corticosteroids, antibiotics and antiviral agents were administered under the diagnosis of meningoencephalitis. His mentality recovered 6 hours after seizure stopping, but looked apathetic in general appearance since then. Electroencephalogram showed intermittent slow waves in right temporal. Brain magnetic resonance imaging (MRI) showed symmetric, bilateral hyperintensities in thalami, hippocampi on T2-weighted image, diffusion weighted image and low ADC coefficient. He developed multiple seizures again on the 7th day after admission. Two weeks after disease onset, follow up MRI revealed the disappearance of previous high signal intensities in involved areas. During the follow up, he showed improvement in mentality and motor functions under rehabilitative treatment. Conclusion : ANE should be in a differential diagnosis whenever patient presents with acute encephalopathy having symmetric thalamic lesions. Reversible thalamic lesions can be present in mild form of ANE.