Unexplained intraoperative coagulopathies continue to be a diagnostic and therapeutic dilemma. The pathophysiology behind unexplained intraoperative coagulopathies is of great variety and complexity (preexisting coagulopathies, dilutional coagulopathy, interactions of medications etc.). We have shown in prospective studies that patients undergoing elective surgery who develop "unexplained" intraoperative coagulopathies have significantly less FXIII per unit thrombin available at any point in time (i.e. already preoperatively) than patients without such coagulopathies. The consequence is a significant loss of clot firmness associated with an increase in intraoperative blood loss. Thus, these patients have less cross-linking capacity to begin with, which explains their preoperatively increased fibrin monomer concentration. The association of increased preoperative fibrin monomer and increased intraoperative blood loss was prospectively evaluated and confirmed in a separate clinical study. It is important to note that the acquired or (compared to the amount of thrombin generated) relative F. XIII deficiency in situations with surgical stress shows early clinical relevance (even if only mild to moderate changes are present); this differs from the experiences with patients with inborn FXIII deficiency, where a pronounced deficiency must be present to have clinically significant spontaneous bleeding. Patients undergoing elective surgery, without clinically obvious coagulopathy but increased preoperative fibrin monomer concentration (as a marker of decreased crosslinking capacity) are at risk for increased intraoperative blood loss. This new concept helps to explain the pathophysiology behind unexplained intraoperative coagulopathies and thus allows for corresponding treatment strategies. Further clinical studies for early detection and interventions in patients with such coagulopathies are necessary.