: The objective of the study was to assess the effect of tissue plasminogen activator administered during catheter-directed thrombolysis (CDT) on coagulation factor XIII (FXIII). Thrombolytic therapy carries significant risks, such as life-threatening bleeds. The mechanisms responsible for major bleeds and intracerebral hemorrhages during thrombolysis are not fully understood. Activated FXIII (FXIII-A) lies at the intersection of coagulation and fibrinolysis. Using purified proteins and blood collected from nine deep vein thrombosis patients undergoing CDT, the stability of FXIII-A and FXIII were measured immediately before, immediately after and 1-day post thrombolysis. We found that purified tissue plasminogen activator directly degraded FXIII-A. During CDT, FXIII levels were decreased by more than 40% in five of nine patients and FXIII-A levels were decreased by more than 85% in two patients when it was activated. FXIII-A and FXIII-A can decrease during CDT in some patients, warranting further research into the role of FXIII-A in bleeding from thrombolysis.