Recent findings have indicated the association between activated protein C (APC)-resistance and cerebrovascular disease. These reports prompted us to investigate whether resistance to APC could be found in patients suffering from transient ischaemic attacks or stroke. Therefore, we studied APC-resistance in 14 young adults belonging to three different families with a history of transient ischemic attacks (TIAs) and stroke. Nine out of fourteen subjects showed APC-resistance but no deficiencies in the anticoagulant proteins AT III, PC and PS. The family history demonstrated a distribution of APC-resistance compatible with dominant autosomal inheritance. A rapid screening method to detect factor V R506Q (Leiden) mutation without sequencing or restriction enzyme digestion has been set-up after biochemical analyses. DNA analysis showed a guanine to adenine transition at nucleotide 1,691 in patients and their relatives with poor response to activated protein C detected by APTT tests. Of 14 investigated subjects and their family members, 5 were normals, 6 were heterozygotes and 3 were homozygotes for factor V mutation. The mutation, in heterozygous form, was also found in 1.3% of our normal population (n = 75). Our findings indicate a possible involvement of APC-resistance in the pathogenesis of arterial thrombosis in young adults.