Achondroplasia is the most common form of dwarfism and has an incidence of approximately 1/7500. In more than 98% of cases, the disease is associated with a G to A or G to C substitution at nucleotide position 1138 (p.G380R) of the fibroblast growth factor receptor 3 (FGFR3) gene. We have developed a sensitive single tube tetra-primer PCR assay to detect both the c.1138G>A and c.1138G>C mutations and can successfully distinguish DNA samples that are homozygous and heterozygous for the c.1138G>A mutation. Titration studies showed that the assay could reliably detect one copy of the mutant allele in a mix of 100 wild-type alleles. The assay has been tested in 50 healthy controls, 3 known patients with achondroplasia, and 5 amniotic fluids suspected of having achondroplasia and for whom we had previously determined the genotypes for the c.1138G>A mutation by PCR-RFLP. We have observed complete concordance between methods. Our tetra-primer PCR assay is sensitive, low-cost, and easy to use method for FGFR3 p.G380R genotyping, which could be used even in "low-tech" laboratories.