A 3 1/2-year-old slightly retarded boy with marked deficiency of arylsulfatase A (ASA) activity in leucocytes and fibroblasts and almost no cerebroside sulfatase (CS) activity in fibroblasts was tested with the sulfatide-loading test. On this test his fibroblasts showed impaired degradation. A pathological excretion of sulfatides was seen in his urine. Nerve conduction velocity, visual evoked potential, auditory brain stem evoked response, and somatosensory evoked potential were all normal. His father and older brother had similarly low levels of ASA in leucocytes and fibroblasts and 1.7-2% residual CS activity in fibroblasts. Although both were clinically normal, their fibroblasts accumulated increased amounts of sulfatides when challenged in the sulfatide-loading test. In this family, this test thus will be of no value in prenatal diagnosis to discriminate among low ASA fetuses with pseudoarylsulfatase A deficiency and fetuses with this unusual ASA deficiency variant.