Objectives Thrombosis is a major contributor to complications associated with coronary interventions. It is unclear whether patients who have undergone angioplasty are predisposed to thrombus formation because of underlying perturbations in their hemostatic equilibrium. Methods Concentration or activity was measured for 14 plasma proteins involved in the coagulation, fibrinolytic, and inhibitory systems. Baseline systemic measurements were compared between patients undergoing balloon angioplasty (n = 15) and normal subjects (n = 32), with sampling repeated at the end of the procedure. To better assess the local hemostatic environment near the site of dilation, intracoronary arterial samples were also obtained just proximal and distal to the dilated stenosis. Results Multiple differences in measured coagulation proteins were found at baseline between the angioplasty candidates and control subjects, including higher mean concentration of plasma fibrinogen (P <.001) and lower high-molecular-weight kininogen concentration (P <.01) and factor XII activity (P <.01). Concentrations of the inhibitory proteins antithrombin III and protein S also differed significantly (P <.001 and P <.01, respectively), with a trend toward lower protein C concentration as well (P <.05). Finally, heightened fibrinolysis was suggested by a marked increase in mean plasma d -dimer concentration in the angioplasty candidates (293 ± 191 ng/mL vs 116 ± 31 ng/mL, P <.01), with a more modest increase in tissue plasminogen activator (P <.05) and decrease in α₂ -antiplasmin (P <.001). Importantly, none of the parameters obtained during the procedure differed significantly from samples obtained before and after angioplasty, and no translesional gradients were observed. Conclusions Patients with active ischemic syndromes who are considered candidates for coronary angioplasty demonstrate significant and multiple alterations in their coagulation, inhibitory, and fibrinolytic systems. However, no further changes were observed during coronary dilation, either systemically or locally, after pretreatment with typical doses of heparin and aspirin. (Am Heart J 1999;137:274-83.)