AIM We present a 3-year follow-up of a boy with mucopolysaccharidosis type II (MPS II) who had idursulfase therapy initiated at the age of 3 months and compare his clinical course to his healthy twin brother. METHODS Detailed anthropometric features, ultrasound studies of liver and spleen volumes, echocardiography and audiological examinations, psychological tests, joint range of motion (ROM) and skeletal radiographs were monitored. RESULTS After 3 years of treatment, the patient has not developed any clinical manifestations of MPS II. He did not develop coarse facial features, joint disease, or organomegaly, and his cardiac function remained normal. There were no pronounced signs of dysostosis multiplex on radiographs. The only difference when compared with his healthy twin brother was lower IQ (Termann-Merrill 98 vs. 118) and mild deformity of one vertebrae. CONCLUSION Our study suggests that early initiation of enzyme replacement therapy may significantly slow or prevent the development of irreversible disease manifestations and therefore modify the natural history of MPS II.