Glanzmann's Thrombasthenia is a rare inherited disorder of platelet aggregation with normal platelet count and humoral coagulation. It is caused by the deficiency or functional disorder of platelet membrane glycoproteins IIb und IIIa. This complex is considered to be a receptor for fibrinogen. Menorrhagia often occurs as a clinical manifestation of affected females. We report a case of severe menorrhagia in a 13-year-old girl during her third menstrual cycle. She needed several red blood cell transfusions. The bleeding could only be stopped by administration of Lynestrenol.