The mutation G1691A (R506Q) in the human factor V gene is associated with the resistance to activated protein C (APC) that represents a major risk of development of venous thrombosis. A population study of 180 unrelated individuals from south Germany was performed. Examination of the allelic frequencies revealed a high prevalence of this disease-related mutation (Q506, FV Leiden). The heterozygosity rate was 7.8% with a confidence interval between 4% and 11%.