A sample of 140 children exhibiting neurologic disturbances (93 suffering from epilepsy and 47 with delayed psychomotor development or mental retardation) was tested for the activity of some lysosomal enzymes. A partial deficiency of arylsulfatase A (ASA) in leucocytes (activities lower than 60% of the control average) was detected in 36 patients (25.7%), whereas few ASA-deficient individuals (1.4%) were found in the control sample of 71 healthy children. Therefore, the frequency of ASA deficiency is abnormally high in our sample of pediatric patients. ASA activity levels were also assayed on fibroblasts from 12 of the 36 ASA-deficient patients; the mean activity in these cells was 20% of the control average. Excretion of urinary sulfatides was not increased in the tested ASA-deficient patients (10/36). Clinical symptoms of these ASA-deficient patients bore no resemblance to classical metachromatic leucodystrophy (MLD), but resemble literature cases labeled as atypical MLD or diagnostic puzzles. This result suggests that reduced ASA activity might be associated with an increased risk of developing neurologic or neuropsychiatric disturbances in children.