Abstract A case of pseudoarylsulfatase A deficiency in an adolescent boy presenting with affective lability, impulsivity, aggression, inattention, and academic difficulties is described. Genetically related to metachromatic leukodystrophy, pseudoarylsulfatase A deficiency has generally been felt to be a benign disorder. Pseudodeficiency of arylsulfatase A has, however, been associated with serious psychiatric morbidity in recent studies. Possible explanations for this association are suggested. To the best of the authors' knowledge, this is the first case report of pseudoarylsulfatase A deficiency in a psychiatrically disturbed adolescent.