Twelve patients with early infancy onset megalencephaly and leukodystrophy with a mild clinical course are reported. The neuroradiological, clinical, and genetic aspects of this recently recognized familial leukodystrophy syndrome were reviewed. Five were affected siblings, and all patients had consanguineous parents. Macrocephaly, a slowly progressive delay in motor development and mild mental deterioration constitute the clinical triad of the disease, showing characteristic age-related onset. The clinical findings outlined remarkably slight functional deterioration despite severe lesions on magnetic resonance imaging (MRI), especially in the initial period. Characteristically, mental function is preserved for years after onset of the motor deficit. The MRI lesions do not reflect the progress of disease. The disease probably has an autosomal recessive mode of inheritance even though no metabolic defect has been detected to date. In a more severe variant of the mentioned disease, there is more progressive and severe neurological dysfunction, including ataxia and spastic quadriparesis, leading to an inability to walk independently after 10 years of age. In mild variants, however, disease severity varies from macrocephaly with near-normal pyschomotor development to mild motor and/or mental dysfunction. Seizures were observed in both types but response to drugs was good. The 12 patients reported here confirm the specific and distinguishing clinical and radiological features of the previously reported 51 cases with this new syndrome, while adding some information regarding identification of the disease.