OBJECTIVE The aim of the study was to determine if Sonoclot with its sensitive glass bead-activated, viscoelastic test can predict postoperative bleeding in patients undergoing cardiac surgery at predefined time points. METHODS A prospective, observational clinical study. METHODS A teaching hospital, single center. METHODS Consecutive patients undergoing cardiac surgery (N = 300). METHODS Besides routine laboratory coagulation studies and heparin management with standard (kaolin) activated clotting time, additional native blood samples were analyzed on a Sonoclot using glass bead-activated tests. Glass bead-activated clotting time, clot rate, and platelet function were recorded immediately before anesthesia induction and at the end of surgery after heparin reversal but before chest closure. RESULTS Primary outcome was postoperative blood loss (chest tube drainage at 4, 8, and 12 hours postoperatively). Secondary outcome parameters were transfusion requirements, need for surgical re-exploration, time of mechanical ventilation, length of intensive care unit and hospital stay, and hospital morbidity and mortality. Patients were categorized into "bleeders" and "nonbleeders." Patient characteristics, operations, preoperative standard laboratory parameters, and procedural times were comparable between bleeders and nonbleeders except for sex and age. Bleeders had higher rates of transfusions, surgical re-explorations, and complications. Only glass bead measurements by Sonoclot after heparin reversal before chest closure but not preoperatively were predictive for increased postoperative bleeding. CONCLUSIONS Sonoclot with its glass bead-activated tests may predict the risk for postoperative bleeding in patients undergoing cardiac surgery at the end of surgery after heparin reversal but before chest closure.