Laron syndrome is a rare autosomal recessive disorder characterized by resistance to growth hormone (GH). In 10 patients of different ethnic origins, we have analyzed all the GH receptor (GHR)-coding exons along with their splice junctions and 6 intragenic polymorphic sites defining several GHR gene haplotypes. This allowed us to identify the mutations in the 20 chromosomes studied and to describe a new GHR haplotype. Eleven different mutations associated with various GHR haplotypes were observed; they included 3 nonsense mutations, 3 splice defects and 5 missense mutations. Of the 11 mutations, 8 were novel. All the mutations involved the exoplasmic domain of the receptor and all the missense mutations were clustered in a short polypeptide segment. Most of the missense mutations affected residues conserved among GHRs from different species and the related molecules that belong to the cytokine receptor superfamily. Adding to the 5 mutations so far described, these findings illustrate the allelic heterogeneity of this syndrome and document the independent origin of the molecular defects, all features of clinical relevance for genetic counselling.