Achondroplasia is the most common genetic form of dwarfism inherited as an autosomal dominant disorder. Individuals affected with achondroplasia have impaired ability to form bone from cartilage (endochondral bone formation). Homozygous achondroplasia is a neonatal lethal condition. The vast majority of patients with achondroplasia have a G-to-A transition at position 1138 of the fibroblast growth factor receptor 3 (FGFR3) cDNA sequence, resulting in the Gly-to-Arg substitution at position 380 of the FGFR3 protein. This mutation has been diagnosed by SfcI digestion of amplified genomic DNA. However, it has also been demonstrated that the SfcI digestion protocol does not consistently distinguish between DNA samples heterozygous and homozygous for the G1138A substitution. This study was designed to improve the molecular diagnosis based on the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) techniques for the FGFR3 G1138A mutation. The newly designed forward primer contains one mismatch (G at position 1136) from the FGFR3 cDNA sequence (A at position 1136), thereby creating a PstI site (CTGCAG at positions 1134 to 1139) in the amplified DNA from alleles containing the G1138A mutation. The PCR-RFLP technique based on the PstI digestion of amplified genomic DNA with a novel forward primer shows 100% accuracy in diagnosis of the G1138A mutation in heterozygous and homozygous individuals.