Mucopolysaccharidosis type II (MPS II - Hunter syndrome) is a rare X-linked recessive disease of lysosomal glycosaminoglycan metabolism leading to a systemic storage disorder. We report three adult brothers (aged 46-52 years) with attenuated Hunter syndrome after 12 months of enzyme replacement therapy with idursulfase. Before enzyme replacement therapy, each had serious complications of their disease: in addition to all requiring urgent cervical spinal canal decompressions in middle age, one required emergency aortic and mitral valve surgery, another had stage IV airways disease, and the third had acute glaucoma resulting in unilateral blindness. One brother discontinued therapy after 12 months. The other two brothers reported subjective improvements in energy and exercise tolerance.