Factor V Leiden was recently found to be the most common cause of familial venous thrombosis in the European population. We have studied the prevalence of factor V Leiden by DNA analysis among 500 Thai blood donors (male 285, female 215). Their ages ranged from 18 to 60 years with a mean of 33 years and 2 months. All of them were healthy voluntary blood-donors who met the standard criteria of the American Association of Blood Banks. No history of thrombosis was found. The results revealed that factor V Leiden was not present among 1,000 chromosomes from Thai blood donors. This suggests that factor V Leiden is not the common genetic predisposing factor of venous thrombosis in the Thai population as compared to the European population.