The deficiency of a lysosomal hydrolase usually results in the storage of its substrate(s) leading to various clinical abnormalities, typical for each deficiency. However, in certain lysosomal hydrolases, an apparent deficiency was noted which does not result in the classical clinical picture. This condition was described for aryl sulfatase A, beta-hexosaminidase, alpha-galactosidase, and galactocerebrosidase, where apparently healthy individuals showed in vitro very low hydrolase activity, usually indistinguishable from the affected patients. The deficiency was usually observed with both the synthetic and natural substrates. In the case of aryl sulfatase A deficiency, no clinical abnormalities were noted in these individuals, and cultured cells obtained from them were able to catabolize normally the natural substrate. Such cases are therefore referred as pseudodeficient. In other cases, such as in beta-hexosaminidase-A deficiency, mild manifestations of the corresponding disorder were reported with subsequent intralysosomal storage of GM2 ganglioside. Our analysis indicates that most of these cases represent a compound heterozygote for the deficient allele and another allele coding for an in vitro low enzyme activity (pseudodeficiency). A complete biochemical explanation for this phenomena is not yet established. The importance of understanding this condition(s) for proper genetic counseling is discussed.