The aetiology of venous thromboembolism in adolescents is frequently associated with hereditary abnormalities of the coagulation system, autoimmune disorders or malignancies. The advent of specific laboratory tests has refined the identification of genetic traits. In this case report, we describe the occurrence of pulmonary embolism in young twins. Intensive tumour screening remained unremarkable. Evaluation of established risk factors for a clotting disorder remained negative, with the exception of a plasminogen activator inhibitor-1 4G/5G polymorphism. Despite the mild association accompanied by the presence of the 4G allele, this polymorphism might predispose to venous thromboembolism in some cases in general and in our case in particular.