In 57 children with congenital cyanotic heart disease coagulation analyses were performed before operation. The patients with low haematocrit only sometimes exhibited defects of low degree. In patients with haematocrit values of more than 60% were increasingly found complex coagulation disturbances. Many correlations could be proved between the coagulation parameters. Thrombocytopenia, plasmatic defects and hyperfibrinolysis were parallel. The cause of the changes is to be seen in chronic disseminated intravasal coagulation processes. The results of investigations presented confirm the necessity of preoperative coagulation analyses especially in patients with high haematocrit values. Larger disturbances of coagulation may influence the decision between a palliative intervention or immediate causal operation.