ABSTRACT. Weissbach, G., Domula, M., Lenk, H. and Schneider, P. (Paediatric Clinic of the Medical Department, Karl‐Marx‐University, Leipzig, DDR). The progressive anti‐thrombin activity and its relations to other factors of the coagulation system in newborns. Acta Paediatr Scand, 63: 555, 1974.—The progressive antithrombin and its relations to other factors of the coagulation system have been studied in chronically asphyxiated newborns. The progressive antithrombin activity was determined in 44 healthy and in 40 chronically asphyxiated newborns according to the method described by Gerendas & Monkhouse. The activity was clearly depressed in the asphyxiated group as compared with healthy full‐term newborns. Only within the group of asphyxiated newborns did the statistical analysis reveal close correlations between progressive antithrombin and the factors fibrinogen, plasminogen, thromboplastin time value, factor II, and thrombocytes. Furthermore, most of these were also closely correlated among themselves. The partly very close relations can be explained only by a simultaneous consumption of these constituents by disseminated intravascular coagulation processes and secondary hyper‐fibrinolysis. The decrease in progressive antithrombin is due to an irreversible binding of the antithrombin III to thrombin, liberated within the vessels.