The detection of homozygous (disease state) and heterozygous (carrier) forms of metachromatic leukodystrophy (MLD) and their prevalence among psychiatric individuals are reviewed. Levels of Arylsulfatase A (ASA) activity in peripheral leukocytes, mixed white cell populations, and lymphocytes are compared in normal and psychiatric patients. The prevalence of low levels of enzyme activity in psychiatric patients, and the implications of such levels with regard to the metabolic disease states and associated psychiatric illnesses are discussed. In addition, the use and reliability of leukocyte enzyme assay systems as a criteria for determining and distinguishing between the homozygous and heterozygous conditions are evaluated.