Megalencephalic leukoencephalopathy with subcortical cysts is one of the newly described white-matter disorders for which recognition has been brought about by advances in imaging technology. The essential diagnostic features include megalencephaly noted in infancy, motor disability in the form of spasticity, ataxia, occasional seizures, mild cognitive decline, and slow progression. Magnetic resonance imaging (MRI) shows bilateral extensive white-matter changes with cysts in the temporal regions. Based on the clinical and MRI features, megalencephalic leukoencephalopathy with subcortical cysts can be distinguished from other conditions (ie, Alexander's disease, Canavan's disease, glutaricaciduria type I) that present in infancy with megalencephaly. Megalencephalic leukoencephalopathy with subcortical cysts is an autosomal recessive disorder, and mutations in the MLC1 gene have now been shown to cause this condition. Several genotypic and phenotypic variations have been described. In India, megalencephalic leukoencephalopathy with subcortical cysts occurs predominantly in the Agarwal community. A common mutation in the MLC1 gene has been seen in 31 Agarwal patients, which suggests a founder effect.