Moyamoya disease is a rare vaso-occlusive illness with an unknown etiology characterized by stenosis of the internal carotid arteries with spontaneous development of a collateral vascular network.A 15-month-old girl was referred to the emergency ward of Imam Reza Hospital due to decreased level of consciousness, focal seizures and fever during the previous 24 hours with an impression of encephalitis. Physical examination revealed left side hemiparesis; however brain CT-Scan did not show any significant lesions. Initial therapy with vancomycin, ceftriaxone and acyclovir was administered. CSF analysis did not show any abnormality and the blood as well as CSF cultures results were negative. Brain MRI showed hyperintensity at right frontal and parietal regions, suggesting vascular lesion. Magnetic resonance angiography (MRA) showed bilaterally multiple torsions in vessels at the basal ganglia consistent with moyamoya vessels.In all children exhibiting encephalitis, vascular events such as moyamoya disease should be considered. Brain MRI is a critical tool for this purpose. Common causes of encephalitis such as herpes simplex should also be ruled out.