We present a 3-year-old boy affected with Hunter syndrome. When we first evaluated the patient glycosaminoglycans (GAG) in urine were elevated (94.6 ng/nmol/creatinine); the enzyme activity determined in serum was decreased (3.9 nmol/hxml) and the mutation found was N350H, exon 8. His clinical signs were coarse facial features, hepatomegaly (6 cm), splenomegaly (6 cm), elbow stiffness and hypospadias; dilatation of the perivascular spaces and white matter abnormalities, mitral regurgitation. After two weeks on enzyme replacement therapy (ERT) with idursulfase (IDS), the excretion of GAG was decreased to 36.2 ng/nmol/creatinine and the liver and spleen volumes were reduced to normal limits. He was subsequently noted to have a softer, finer skin, he had no further bouts of bronchitis, and his physical activity improved. This indicates that IDS in young children is well tolerated and that it has several effects which may confer clinical benefits with long-term therapy.