The mechanism of thrombophilia in patients with antiphospholipid antibodies (aPL) is not clearly understood. A number of contributing factors have been described, but more than one may be operative. It was recently found that aPL may cause the acquired activated protein C resistance phenotype, whereas in familial thrombophilia, activated protein C resistance frequently results from a point mutation in the factor V gene (replacing arginine 506 with a glutamine) that leads into the (R-506-Q), the so-called Leiden mutation, that produces a mutated factor V, resistant to the catalytic action of activated protein C, otherwise normal in its procoagulant properties. We describe one patient heterozygous for the activated protein C resistance genotype with a familial form of primary antiphospholipid syndrome, who had a nephew who died with this disease. In individuals who are heterozygous for the factor V Leiden mutation, the presence of aPL may cause further activated protein C resistance, resulting in increased thrombophilia.