Metachromatic leukodystrophy (MLD) is a rare lysosomal sphingolipid storage disorder, caused by a deficiency of arylsulfatase A (ASA). It is inherited in an autosomal recessive way, among Caucasians three causing alleles are frequent. Demyelination is the hallmark of MLD. Interest in the disease has increased as therapeutic options such as stem cell transplantation, enzyme replacement and gene therapy are topics of current research. A late-infantile (onset before 3 years of age), a juvenile form (onset before 16 years) and an adult form are usually distinguished. Rapid motor decline is typical for the first and also the second forms, the second may be preceded by cognitive and behavioural problems, which mainly characterize the adult form. There is evidence for a genotype-phenotype correlation: patients homozygous for alleles which do not allow the expression of any enzyme activity (null-allele) suffer from the late infantile form; heterozygosity for a null allele and a non-null allele are more associated with the juvenile form and homozygosity for non-null alleles is more frequent in the most attenuated adult onset form.