Mucopolysaccharidosis type I (MPS-I) is an autosomal recessive disease caused by mutations in the alpha-L-iduronidase (IDUA) gene. These mutations lead to a deficiency of the glycosidase alpha-L-iduronidase (IDUA), which is required for the degradation of heparan sulphate and dermatan sulphate and thus the storage of these glycosaminoglycans in the lysosome. There is a wide range of clinical phenotypes in MPS-I (eponyms: Hurler syndrome, severe; Hurler/Scheie syndrome, intermediate; Scheie syndrome, mild), which makes prediction of disease severity and genetic counselling difficult. However, since cloning of the IDUA gene, mutation analysis has provided some molecular explanations for the range of MPS-I phenotypes, in turn facilitating the selection and evaluation of patients undergoing experimental treatment protocols such as bone marrow transplantation. A total of 46 mutations now have been defined for MPS-I consisting of 8 nonsense mutations, 21 missense mutations, 3 splice site mutations, and 14 minor deletions and/or insertions. Furthermore, 30 polymorphisms or nonpathogenic sequence variants have been defined, including 7 amino acid substitutions. Among patients of European origin, there are two major MPS-I mutations and a number of less frequent mutations. It is possible to follow mutation analysis of 292 patients, which can be divided into eight main patient groups of different ethnic and/or geographic origin with significant variation in mutant allele frequencies. A complex picture of molecular heterogeneity is emerging, building a valuable database for genotype/phenotype correlation. Mutation analysis is also providing some of the first clues into the structure and function of IDUA.