It has been reported that mutations in the FGFR3 gene cause autosomal dominant forms of dwarfism, achondroplasia (ACH) and hypochondroplasia (HCH). In the present study, we analyzed the FGFR3 gene in 26 Japanese patients with ACH and 14 with HCH. Genomic DNAs of the patients were isolated from whole blood. For the ACH patients, a 164-bp fragment of the FGFR3 gene that spans the entire transmembrane domain was amplified by polymerase chain reaction (PCR), and the PCR products were analyzed by direct sequencing of the PCR products and by digestion of the PCR products with restriction enzymes. For the HCH patients, a 206-bp fragment of the FGFR3 gene which encodes a part of the TK1 domain was amplified, and the PCR products were directly sequenced. The heterozygous G380R mutations were identified in all of the 26 ACH patients, whereas the heterozygous N540K mutations were identified in 8 out of 14 HCH patients. These results were consistent with previous reports from abroad.