Mucopolysaccharidosis (MPS) includes a group of lysosomal storage diseases. Among them, type III (Sanfilippo) disease is rarely described in Taiwan because of their complexity. With accurate quantifying and precise separation of urinary glycosaminoglycans (GAGs), and specific enzyme measurements, two cases of MPS IIIB disease were able to be described. They both had mild-to-moderate degrees of mental retardation, facial dysmorphism and dysostosis multiplex which do not differ from other types of MPS. Total amounts of GAGs in the urine were only mildly elevated but, among them, heparan sulfate was the highest. Skin fibroblasts alpha-N-acetyl-glucosaminidase activities were low in both cases. Therefore, analysis of GAGs and enzyme assays are important for the diagnosis of patients suspected to have MPS, especially type III disease.