A child with a diagnosis of late-infantile metachromatic leukodystrophy (MLD), and a normal father with low arylsulfatase A (ASA) activity in leucocytes and cultured fibroblasts is described. The child had a pathologically increased amount of sulfatides in the urine, whereas no sulfatides could be found in the father's urine. Sulfatide-loading of the child's cultured fibroblasts showed an accumulation of sulfatides, whereas the fibroblasts from the father had a marginally decreased sulfatide turnover. It is thus possible to discriminate between these two forms of low ASA activity in this family, and to ensure a correct diagnosis should the amniotic fluid cells show a low ASA activity in future pregnancies.