In a four and a half-year-old boy with a progressive cerebral process in his last year of life the arylsulfatase A (ASA) and cerebroside sulfatase activity was lowered to values only slightly above those of metachromatic leukodystrophy (MLD) homozygotes. Morphologically, however, the brain showed a diffuse-disseminated sclerosis without any findings of MLD. The enzyme findings including the reduced ASA values in both parents suggested that our proband was a mixed heterozygote both of MLD and pseudo-ASA-deficiency, but casually suffered from that inflammatory demyelinating process. An additional biochemical finding was the moderate increase of brain sulfatides which can be seen in relation to the mixed heterozygosity for ASA deficiency.