This study determined the prevalence of inherited factor V Leiden mutation in a group of 128 thrombosis patients (102 with venous thrombosis and 26 with arterial thrombosis) attending a hospital in Sousse, Tunisia, and a control group of 100 with no history of thrombosis. Using an allele-specific PCR amplification technique, factor V Leiden was found in significantly more patients (20.3%) than controls (6.0%). The higher prevalence was significant in the subgroup of venous thrombosis patients but not in arterial thrombosis patients. The allele frequency was 3.5% in the normal Tunisian population. Screening Tunisian patients with venous thrombosis and their relatives for factor V Leiden may be justified.