Acquired von Willebrand syndrome (AvWS) is a rare bleeding disorder associated with a number of different diseases, including hypothyroidism. We describe two prepubertal girls with AvWS and undiagnosed hypothyroidism due to thyroiditis Hashimoto. The patients had neither family history nor symptoms of bleeding disorders. Substitution therapy with levothyroxine led to normalization of the coagulation parameters. We report these cases in order to raise awareness among paediatricians so that the AvWS should be suspected and searched for with the appropriate laboratary tests in all cases of hypothyroidism. Moreover, patients with bleeding diathesis of unknown origin should also be investigated for hypothyroidism.