Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused by the deficiency of arylsulfatase A (ASA). A substantial ASA deficiency has also been described in clinically healthy persons, a condition for which the term pseudodeficiency was introduced. The discrimination of both kinds of deficiencies based on ASA activity determination is difficult and unreliable. This creates a serious problem in the genetic counseling and diagnosis of MLD. The mutations characteristic for the pseudodeficiency (PD) allele have recently been identified. A non-radioactive assay based on the polymerase chain reaction is described, which allows the rapid detection of the ASA pd allele. The assay utilizes pairs of primers that allow either the amplification of the ASA PD allele or of other ASA alleles, since their 3' residues match either the ASA PD allele or other ASA alleles.