Abstract Sanfilippo syndrome or mucopolysaccharidosis III (MPS III), includes a group of four autosomal recessive lysosomal storage disorders caused by deficient activity of enzymes involved in the catabolism of heparan sulfate. The four types of MPS III are recognized in accordance with the deficient enzyme, resulting in the accumulation of heparan sulfate with particularly deleterious effects in the central nervous system. The incidence of MPS III remains to be established in Latin American countries. We describe the journey of a patient with MPS IIIB whom, even in the presence of speech delay and deterioration, behavioral problems and motor incoordination, showed unaltered urinary glycosaminoglycans (GAGs) levels. An investigation for MPS was undertaken and enzyme analysis indicated a deficiency of alpha-N-acetylglucosaminidase, leading to the diagnosis of MPS IIIB. With the correct diagnosis, the patient’s symptoms could be properly managed, and the parents received appropriate genetic counseling. The present case report reinforces the need of investigating MPS III in patients with language delay and/or regression, neurological impairment and behavioral alterations, even when urinary GAGs are within normal range. A definitive diagnosis ends the diagnostic journey and enables the medical team and family to provide a better care for the child.