Objective: To describe a novel pattern of magnetic resonance imaging (MRI) abnormalities as well as the associated clinical and laboratory findings. Design: The MRIs of more than 3000 patients with an unclassified leukoencephalopathy were systematically reviewed. Clinical and laboratory data were retrospectively collected. Setting: University hospital. Patients: Seven patients (3 male) shared similar MRI abnormalities and clinical features. Main Outcome Measures: Pattern of MRI abnormalities and clinical and laboratory findings. Results: The MRIs showed signal abnormalities of the deep cerebral white matter, corpus callosum, thalamus, basal ganglia, brainstem, and cerebellar white matter between the ages of 9 months and 2 years. On follow-up, abnormalities gradually improved. Clinical regression occurred in the second half-year of life with spasticity and loss of milestones. From the second year on, clinical improvement occurred. So far, no second episode of regression has happened. Lactate levels were elevated during clinical regression. Conclusion: These patients represent a single novel leukoencephalopathy, probably caused by a mitochondrial defect. Arch Neurol. 2012; 69(6): 718-722. Published online February 6, 2012. doi:10.1001/archneurol.2011.1048