As assay for the detection in leukocytes of homozygous and heterozygous carriers of Sanfilippo syndrome type C is described. In one family with two patients suffering from Sanfilippo C syndrome, the affected individuals had no residual activity to acetyl-CoA: alpha-glucosaminide N-acetyltransferase. The determination of the acetyl-CoA: alpha-glucosaminide N-acetyltransferase/ beta-glucuronidase ratio allows the discrimination between obligate heterozygotes and normal individuals and may be used for carrier detection.