The objective of this open-label, single arm, monocentric, phase I/II clinical study is to assess safety and efficacy of ARSA gene transfer in the brain of children affected with early onset forms of Metachromatic Leukodystrophy (MLD). For this purpose, an adeno-associated virus serotype rh.10 (AAVrh.10) vector will be used to transfer the ARSA cDNA coding for Arylsulfatase A (ARSA) enzyme into the brain of children. Five patients with early onset form of MLD, age ranging from 6 months to 4 years, will be included in this protocol and will be followed during 24 months. Patients will be selected at presymptomatic or early stage of their disease, following clinical, neuropsychological and brain imaging criteria. Twelve simultaneous injections of the investigational medicinal product will be performed in the white matter of both brain hemispheres, through 6 image-guided tracks, with 2 deposits per track. A low dose (1x10EXP12 vg total) will be administered to the first 2 patients, while the last 3 will receive a higher dose (4x10EXP12 vg total). Safety and efficiency will be evaluated based on clinical, neuropsychological, radiological, electrophysiological and biological parameters.