AbstractMucopolysaccharidosis (MPS) type I is a lysosomal storage disorder caused by deficiency of the enzyme α‐l‐iduronidase (IDUA), which presents with a wide spectrum of phenotypes. Recently, enzyme replacement therapy (ERT) became available for patients with MPS I and has been demonstrated to be safe and effective in patients with the milder Hurler–Scheie and Scheie phenotypes. Treatment for 26 weeks with recombinant human IDUA (laronidase) has been shown to significantly increase the percentage of predicted normal forced vital capacity and the distance walked in the 6‐minute walk test. There was also a clear reduction in the volume of the liver and the levels of urinary glycosaminoglycan excretion. The drug was generally well tolerated. There were no drug‐related severe adverse events, and although the majority of patients developed IgG antibodies, these declined by the end of the study. Conclusion: ERT seems to be a very promising new therapeutic regimen for patients with MPS I, especially for those with the less severe variants. However, as laronidase does not cross the blood–brain barrier it will probably not influence the central nervous manifestations in the most severely affected patients with the Hurler phenotype, although it may improve general lung and heart function, making bone marrow transplantation easier to tolerate.