Hypochondroplasia (MIM 146000) is an autosomal dominant skeletal dysplasia with skeletal features similar to but milder than those seen in achondroplasia^1–4. Within the past year, the achondroplasia locus has been mapped to 4p16.3 (refs 5–7) and mutations in the fibroblast growth factor receptor 3 (FGFR3) gene have been identified in patients with the disorder^8,9. More than 95% of 242 cases reported so far are accounted for by a single Gly380Arg mutation^8–11. McKusick et al.¹² proposed that achondroplasia and hypochondroplasia are allelic based on the similarities in phenotype between the two disorders and the identification of a severely dwarfed individual whose father had achondroplasia and whose mother had hypochondroplasia. There is also genetic linkage evidence that hypochondroplasia and achondroplasia map to the same locus^6,13. We therefore began a systematic screening of FGFR3 to detect mutations in patients with hypochondroplasia. We now report a single FGFR3 mutation found in 8 out of 14 unrelated patients with hypochondroplasia. This mutation causes a C to A transversion at nucleotide 1620, resulting in an Asn540Lys substitution in the proximal tyrosine kinase domain.