Leigh's disease, or subacute necrotizing encephalomyelopathy (SNE), in adults is rare, and its diagnosis has depended on the postmortem identification of characteristic lesions in a typical distribution. We observed an autopsy-proved case of SNE in which the diagnosis was established by the distribution and evolution of lesions documented by serial magnetic resonance imaging (MRI). A 21-year-old woman insidiously developed diplopia and gait disturbance, and subsequently deteriorated to a vegetative state over seven months. An initial MRI obtained one month after presentation showed increased signal intensity that surrounded the aqueduct of Sylvius and involved the tectum of the midbrain. Serial MRI scans showed these lesions to extend and symmetrically involve the tectum of the midbrain, caudate, putamen, globus pallidus, and substantia nigra, while sparing the mammillary bodies and red nuclei. Despite treatment with 2 g of thiamine administered intravenously daily, she continued to deteriorate and died. Results of an autopsy established the diagnosis of SNE and confirmed the MRI-identified distribution of lesions. To our knowledge, this case is the first report of MRI findings in an adult with autopsy-proved SNE, suggesting that MRI can be valuable in the early diagnosis of this disease.