The authors have developed a simple and rapid amplification-refractory mutation system (ARMS) assay for the factor V Leiden mutation (MIM 227400.0001). Polymerase chain reaction (PCR) primers within exon 10 of the factor V gene were designed; one common upstream primer was paired with either a normal or a mutant-specific downstream primer. The assay was validated using factor V Leiden DNA samples of thrombophilic patients ascertained by the restriction enzyme cleavage method of reference. In the normal French population, allele frequencies of the factor V Leiden mutation are 2.6% for the Parisian region and 1.7% for the south of the country.