Background: Acquired deficiency of FXIII because of perioperative hemodilution has been described several times in adults; however, data in children are scarce. We performed a prospective observational trial to evaluate the intraoperative course of FXIII in children undergoing elective major surgery. Methods: Blood samples were repeatedly taken from 46 children aged 0.3-16 years undergoing major surgery. Concentrations of FXIII and fibrinogen, thrombelastometry by ROTEM(®) , and cell count were assessed intraoperatively. Results: A significant decrease in FXIII concentration (median 60%; IQR 49-69%) was already noted at beginning of surgical procedures, while most ROTEM(®) traces remain unchanged. FXIII levels further deteriorated intraoperatively to minimal levels of 33% (15-61%). Lowest intraoperative clot strength (ExTEM) was 44 mm (34-50 mm), and fibrinogen plasma levels decreased to minimal levels of 130 mg·dl(-1) (95-160 mg·dl(-1) ). In 43 of 46 children, transfusion therapy was necessary. Despite of transfusion of fresh frozen plasma (cumulative total dose 22 ml·kg(-1) [11-32 ml·kg(-1) ]) in 21 of 46 children, FXIII level remains low in all children till the end of surgery at levels of 39% (20-46%). Conclusions: Coagulation factor XIII decreased early during major surgery owing to hemodilution. Overall intraoperative FXIII levels remain low despite of transfusion of fresh frozen plasma.