In a family with a metachromatic leukodystrophy patient, two further pregnancies at risk were monitored by amnion cell culture. In one case, a normal baby was predicted and born. In the other case, a prenatal deficiency of arylsulfatase A was found. The diagnosis of metachromatic leukodystrophy was confirmed biochemically in various organs of the fetus by the deficiency of arylsulfatase A. The residual enzyme activity was shown to have an abnormal pH optimum and an increased heat stability. Ultrastructural studies revealed lipid storage in the myelinating nervous system and in the liver. For the interpretation of morphological results, it was indispensable to analyze an age-matched control fetus.