Trauma patients are at a high risk of both bleeding and thromboembolism. This study assessed whether conventional coagulation blood tests were reliable predictors of an increased in vitro thrombotic and bleeding tendency of trauma and non-trauma patients. Conventional coagulation blood tests and thromboelastographs of 63 trauma and 63 randomly selected, critically ill non-trauma patients were compared. Increased in vitro thrombotic and bleeding tendencies were defined by a maximum amplitude >72 mm or an angle >74° on the thromboelastograph and a maximum amplitude < 54 mm or an angle <47°, respectively. In vitro thrombotic tendency was more common than bleeding tendency and this was not different between the critically ill trauma and non-trauma patients (59% versus 67% with thrombotic tendency, P=0.461; 11% versus 10% with bleeding tendency, respectively, P=0.999). Thrombocytopenia (<150 × 10⁹ /l) and low fibrinogen concentrations (<2 g/l) were the only two factors associated with an increased in vitro bleeding tendency (both P=0.001) and thrombocytopenia was the only factor associated with a lower risk of in vitro thrombotic tendency (21% versus 75%, P=0.001). Platelet counts (Pearson's correlation coefficient [ r]: 0.59, P=0.001) and fibrinogen concentrations ( r 0.61, P=0.001) both had a relatively linear association with maximum amplitude of the thromboelastograph. Prolonged International Normalized Ratio (>1.5) and activated Partial Thromboplastin Time (>40 seconds) were, however, not significantly associated with an increased in vitro thrombotic or bleeding tendency. In conclusion, in vitro thrombotic tendency was more common than bleeding tendency in critically ill trauma and non-trauma patients. Platelet counts and fibrinogen concentrations were better predictors of increased in vitro thrombotic and bleeding risks than International Normalized Ratio or activated Partial Thromboplastin Time.