Mucopolysaccharidosis type II (MPSII), or Hunter syndrome, arises from a deficiency in iduronate 2-sulfatase (IDS), and it is characterized by progressive somatic and neurological involvement. The MPSII mouse model reproduces the features of MPSII patients. Systemic administration of the AAV2/5CMV-hIDS vector in MPSII mouse pups results in the full correction of glycosaminoglycan (GAG) accumulation in visceral organs and in the rescue of the defects and GAG accumulation in the central nervous system (CNS). Remarkably, in treated MPSII animals, this CNS correction arises from the crossing of the blood-brain barrier by the IDS enzyme itself, not from the brain transduction. Thus, we show here that early treatment of MPSII mice with one systemic injection of AAV2/5CMV-hIDS results in prolonged and high levels of circulating IDS that can efficiently and simultaneously rescue both visceral and CNS defects for up to 18 months after therapy.