['Hypercoagulability may compromise the patency of bypass grafts. The authors hypothesized that perioperative in vitro platelet responses to varying agonists (eg, thrombin, platelet activating factor, collagen, adenosine diphosphate) correlate with early graft thrombosis after off-pump coronary artery bypass grafting (OPCAB).', 'Prospective study of 78 OPCAB patients with 151 venous bypass grafts treated with perioperative aspirin and intraoperative heparin (250 U/kg).', 'Tertiary, academic medical center.', 'None.', 'Hypercoagulability, defined by TEG (maximum amplitude [MA]>70 mm), whole-blood aggregometry (>15 ohms after 5 mcl/mL collagen) or hemoSTATUS (Ch5CR>0.5), was serially assessed around OPCAB. An immediate decline in platelet function after surgery and on postoperative day 1 returned to normal by postoperative day 3 in most patients. Graft blood flow was analyzed intraoperatively, and vein biopsies were analyzed for endothelial disruption. Graft patency was assessed by multichannel computed tomography coronary angiography on postoperative day 5. No differences in any of the platelet function assays were noted for the 8 patients with graft thrombosis (n=8 grafts) versus the 68 patients with all patent grafts (n=129 grafts). Ten patients developed a rise in platelet function postoperatively >1 SD above baseline; only 1 developed graft thrombosis (p=not significant v patients with normal platelet function).'] Does hypercoagulability affecting early vein graft patency exist after off-pump coronary artery bypass?