Introduction PREVIOUS STUDIES in this series have shown that there is an increase in sulfatides in metachromatic leukodystrophy (MLD).1,10Sulfatide deposits occur not only in relation to the devastated myelin of the central and peripheral nervous system but also in some systemic organs, notably in the kidney and gallbladder. One reason for an increase in sulfated molecules might be a genetically-determined deficiency of certain sulfatase enzymes. Our recent studies have shown evidence of low sulfatase activities in MLD.11,13-15This finding of a sulfatase deficiency in a "model" lipid storage disease has been doubly intriguing. Not only does it raise the possibility of some cause and effect relationship with the sulfatide lipidosis of MLD, but it also serves to focus attention on some critical, if thorny, biomedical problems. These concern the genetic regulation, subcellular location, and the broader metabolic role of the hydrolytic enzyme systems in general. Evidence reviewed