We have identified a novel fibroblast growth factor receptor 3 (FGFR3) missense mutation in four unrelated individuals with skeletal dysplasia that approaches the severity observed in thanatophoric dysplasia type I (TD1). However, three of the four individuals developed extensive areas of acanthosis nigricans beginning in early childhood, suffer from severe neurological impairments, and have survived past infancy without prolonged life-support measures. The FGFR3 mutation (A1949T: Lys650Met) occurs at the nucleotide adjacent to the TD type II (TD2) mutation (A1948G: Lys650Glu) and results in a different amino acid substitution at a highly conserved codon in the kinase domain activation loop. Transient transfection studies with FGFR3 mutant constructs show that the Lys650Met mutation causes a dramatic increase in constitutive receptor kinase activity, approximately three times greater than that observed with the Lys650Glu mutation. We refer to the phenotype caused by the Lys650Met mutation as "severe achondroplasia with developmental delay and acanthosis nigricans" (SADDAN) because it differs significantly from the phenotypes of other known FGFR3 mutations.