Background: Acquired von Willebrand Syndrome (AvWS) is a rare bleeding disorder associated with various underlying conditions. Many case reports have been published so far on bleeding tendency in hypothyroidism resembling AvWS. Objective: This study was designed to define the relationship between hypothyroidism and AvWS and to investigate the effects of Lthyroxine treatment. Subjects: Twenty four hypothyroid patients were included in the study. Nineteen patients were evaluated during treatment, 5 patients were evaluated before hormone replacement. Methods: Complete coagulation screening tests including levels of von Willebrand Factor antigen (vWF:Ag) and functional activity (vWF:RCo) were measured by thrombocyte aggregometer.Results: We demonstrated low vWF:Ag and vWF:RCo in 13 patients. Two of the 13 patients were diagnosed as AvWS, while another 2 patients were diagnosed as hereditary vWD Type 1. The remaining patients are still being followed-up. Conclusion: We would like to attract the attention of paediatricians to the possibility of bleeding due to decreased activity of vWF in hypothyroid children.