The authors describe a girl with mucopolysaccharidosis type IIIB (Sanfilippo disease). She presented with speech delay, macrocephaly, and left lower limb hypoatrophy. Her brain and spinal cord imaging revealed diffuse cystic brain lesions and hemiatrophy of her spinal cord on the left (thoracic levels 11/12). She had marked reduction of her α-N-acetylglucosaminidase activity assay consistent with the diagnosis of mucopolysaccharidosis type IIIB. Mutational analysis showed 2 mutations on exon 6 of the α-N-acetylglucosaminidase gene, both of which were identified in her parents. At 10 years of age the girl had minor learning difficulties and mild behavioral problems. Her spinal cord hemiatrophy, in association with mucopolysaccharidosis type III, has not previously been described in the literature.