We have analysed 3380 chromosomes (1690 unrelated individuals) from twenty-four populations for the presence of factor V Leiden, an important risk factor in venous thromboembolism. The allele frequency in 618 Europeans was 4.4%, with the highest prevalence among Greeks (7%). It was 0.6% in Asia Minor. Factor V Leiden was not found in any of 1600 chromosomes from Africa, Southeast Asia, Australasia, and the Americas. This distribution may partly explain the rarity of thromboembolic disease in these populations. The high prevalence in Europeans suggests that screening for this mutation should be considered in some circumstances.