Patients with congenital heart disease (CHD) often do have a variety of coagulation abnormalities that results in bleeding diathesis. Our study aimed to determine the impact of cyanosis and CHD on modified thrombelastography parameters, compared with children without CHD. Preoperative blood samples were taken for TEM analyses from a total of 51 infants scheduled for surgery. The following groups were examined: normal patients without CHD, acyanotic patients with acyanotic CHD, and cyanotic patients with CHD and with preoperative hemoglobin values higher than 15 g dl(-1). Mean values of all patient groups as well as all individual values of normal patients were within their normal ranges. Within these limits, however, clots were significantly inferior in cyanotic patients (worse mean values of eight out of 10 measured TEM parameters representing the intrinsic, extrinsic, and plasmatic pathways of coagulation) and in acyanotic patients (two out of 10 TEM parameters). Individually, pathological TEM parameters were found in seven (41%) cyanotic patients (P=0.003; vs. normal patients) and in three (17%) acyanotic patients (P=0.01). More than one abnormal TEM coagulation parameter was found in four patients, all of them cyanotic patients. Hyperfibrinolysis was detected in one patient, a cyanotic patient. The present investigation confirms previous findings that in patients with CHD the heart defect itself compromises coagulation monitored with TEM, but in addition, we demonstrate that cyanosis and/or polycythemia exert the essential negative impact on hemostasis. Preoperative hyperfibrinolysis detected with TEM seems to play no important role.