Priapism is a rare disease that can be secondary to a clotting disorder. The authors report the first published case of priapism due to a heterozygous mutation of factor V Leiden responsible for activated protein C resistance in a patient treated with low molecular weight heparin. A similar case has already been reported, but with a homozygous mutation. Priapism must therefore be considered to be a thromboembolic event and the presence of a clotting disorder should be systematically investigated in the absence of an obvious aetiology.