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Background and Purpose: Leukoencephalopathies of unknown origin constitute a considerable problem in child neurology. The purpose of our ongoing study of the subject was to define new disease entities among them by using primarily MR imaging pattern recognition., Methods: We identified seven unrelated patients with a distinct MR imaging pattern consisting of hypomyelination and atrophy of the basal ganglia (neostriatum) and cerebellum (H-ABC). We reviewed the clinical, MR imaging, MR spectroscopic, and laboratory data., Results: Clinically, the patients' diseases were characterized by variably disturbed early development followed by increasing extrapyramidal movement abnormalities, ataxia, and spasticity. Mental capacity was variably affected, but it appeared to be relatively preserved. Parents were not related, and none of their siblings were affected. No metabolic defect was found. Follow-up MR imaging demonstrated atrophy of the cerebral white matter, neostriatum, and cerebellum, which was most pronounced in the most clinically severe cases. Single-voxel proton MR spectroscopic results were normal in the parietal cortex. In the cerebral white matter, myo-inositol and creatine levels were elevated; this finding was compatible with gliosis. N-acetylaspartate and choline levels were normal, suggesting that neither axonal loss nor active demyelination occurred. Proton MR spectroscopic imaging revealed relatively decreased N-acetylaspartate levels in the frontal region., Conclusion: The uniform and highly characteristic MR imaging findings, in combination with the similarities in the clinical findings, provide evidence of a distinct nosologic entity. The acronym H-ABC is offered to indicate patients sharing these clinical and MR imaging features.