The initiation, modulation, and resolution of thrombus associated with eroded or unstable coronary plaques are critically dependent on the efficacy of endogenous fibrinolysis. This is dependent on the cellular function of the surrounding endothelium and vascular wall. In particular, the acute release of tissue plasminogen activator from the endothelium makes an important contribution to the defense against intravascular thrombosis. Here, we describe the rationale and methodology for, and clinical relevance of, assessing acute endothelial tissue plasminogen activator release in humans. The investigation of endothelial fibrinolytic function has the potential to provide major new insights into the pathophysiology of cardiovascular disease, and to shape future therapeutic interventions.