57 patients with cyanotic congenital heart diseases were studied. 39 patients presented t. of Fallot, 18 patients had patent ductus arterious with pulmonary hypertension and right to left shunt. The average whole blood viscosity in patients with t. of Fallot was 6.86+/-0.32 cps. This parameter was correlated with hematocrit, fibrinogen, euglobulin lysis time, pulmonary output, systemic output, and O2 saturation. The value of "r" resulted as statistically significant only for the correlation between viscosity and hematocrit and between viscosity and fibrinogen (t=13.22, p less than 0.0005; t=3.35, p less than 0.001 respectively). The average value of plasma viscosity in patients with t. of Fallot was 1.61+/-0.024 cps. The correlations of this parameter with fibrinogen, euglobulin lysis time, pulmonary output and systemic output did not result as statistically significant. The average fibrinogen value was 256.10+/-20.63 mg%, whereas that of euglobulin lysis time was 12,67+/-1.50 hours. In patients with patent ductus arteriosus the average blood viscosity was 6.65+/-0.56 cps. Such parameter was correlated with hematocrit, fibrinogen, euglobulin lysis time, pulmonary output, systemic output and O2 saturation. The value of "r" resulted as statistically significant only between blood viscosity and hematocrit and between blood viscosity and O2 saturation (t=9.30, p less than 0.0005; t=5.800, p less than 0.001 respectively). The average value of plasma viscosity was 1.61+/-0.04 cps. This parameter was correlated with fibrinogen, euglobulin lysis time, pulmonary output and systemic output. The correlation index "r" never resulted as statistically significant. The average fibrinogen value was 299.20+/-20.30 mg. The average euglobulin lysis time was 16.02+/-2.73 hours.