Cardiopulmonary bypass may alter the factors responsible for normal hemostasis by exposing blood to nonendothelial surfaces, for example, extracorporeal circuits. To evaluate the probable effect of extracorporeal circulation on hemostasis, we measured thrombin-antithrombin III complex (TAT) and prothrombin fragment 1+2 (PF 1.2) plasma levels, which are the biologic markers of in vivo coagulation, in 20 patients who underwent coronary artery bypass grafting (CABG). Postoperative PF 1.2 levels were higher in comparison to the preoperative concentrations of the parameter. Preoperative and postoperative TAT concentrations showed no statistically significant difference. The increment in the PF 1.2 levels in CABG patients might suggest ongoing subclinical hemostatic activation associated with CABG. Further investigations are needed to clarify the exact relationship between increased PF 1.2 levels and thrombotic complications observed in CABG patients. Hemostatis in CABG is still an enigma and remains to be eluciated.