Disseminated intravascular coagulation in association with arterial aneurysm has been reported in several cases. Reports have suggested a continuous deposition of fibrin and/or platelets at the site of the aneurysm followed by fibrinolysis as responsible for this disorder. To further investigate this theory we studied the in vivo binding of an indium-111-labelled monoclonal antibody against human tissue plasminogen activator (t-PA), a proteolytic enzyme involved in the fibrinolysis. Six patients admitted to the hospital for elective resection of an abdominal aortic aneurysm with a mural thrombus were examined. One day before the operation the antibody was injected intravenously. Scintigrams acquired just prior to operation and tissue samples obtained at the operation revealed an increased t-PA accumulation in the wall of the aneurysm. The study demonstrates in vivo, focally increased fibrinolytic activity in the aneurysm, explaining the coagulopathy observed in these patients.