Excessive perioperative bleeding remains a substantial problem. Factor XIII (FXIII) contributes to clot stability, and it has therefore been suggested that supplementation with FXIII concentrate may improve perioperative hemostasis. We evaluated the effects of increasing doses of FXIII, alone or in combination with fibrinogen or platelet concentrate, in blood samples from 2 considerably different groups of surgical patients: cardiac and scoliosis surgery patients. Whole-blood samples were collected immediately after operation from cardiac and scoliosis surgery patients. The samples were supplemented with 3 clinically relevant doses of FXIII concentrate (+20%, +40%, and +60%), alone or in combination with a fixed dose of fibrinogen concentrate (+1.0 g/L) or fresh apheresis platelets (+92 × 10/L). Clot formation was assessed with rotational thromboelastometry (ROTEM). When the highest dose of FXIII concentrate was added, EXTEM clotting time was shortened by 10% in both cardiac and scoliosis surgery patients (95% confidence intervals: 2.4%-17% and 3.3%-17%, respectively), and FIBTEM maximum clot firmness was increased by 25% (9.3%-41%) in cardiac patients, relative to baseline. When fibrinogen was added, the dose-dependent effect of FXIII on clot stability was maintained, but the total effect was markedly greater than with FXIII alone, +150% (100%-200%) and +160% (130%-200%) for the highest FXIII dose in cardiac and scoliosis patients, respectively. Ex vivo supplementation with clinically relevant doses of FXIII improved clot formation moderately in blood samples from cardiac and scoliosis surgery patients, both alone and when given in combination with fibrinogen or platelet concentrate.