Arylsulfatase A (ASA) pseudodeficiency (Pd) was defined as the in vitro measurement of low enzyme activity in a healthy person. A variable incidence of the Pd allele was found in different populations; it was 10-20 times higher than that of metachromatic leukodystrophy (MLD). In Poland we estimated the incidence of the Pd allele at 6% and that of isolated 1788 mutation (loss of glycosylation site) at 3%. Out of 8 cases with neurological symptoms and low ASA activity, 2 were found to be homozygous for the Pd allele; 2 MLD patients had healthy siblings homozygous for the Pd allele and another patient's allele bore two mutations, Pd and that causing MLD.