Dynamic coagulation-thrombus formation studies and evaluation of the aggregation of red cells, plasma viscosity and blood viscosity were carried out on 22 patients with haemophilia A and 6 patients with Christmas disease. Significant differences are found in the formation and viscosity of artificial red/white and white thrombi between patients with haemophilia A and with Christmas disease.A treatment by plasma infusion or by cryoprecipitate leads to an elevation of plasma viscosity and viscosity of artificial thrombi, and to a decrease of the thrombus formation time. Both types of treatment can be considered equivalent from the rheological point of view. Highly significant correlations are established between the fibrinogen levels and all blood viscosity factors. In particular, fibrinogen level and viscosity of artificial thrombi (p < 0.001), fibrinogen and plasma viscosity (p < 0.001), and fibrinogen and aggregation of red cells (p < 0.01).