15 newborn infants with the hyperviscosity syndrome due to polycythaemia i.e. a central haematocrit of at least 65% and a raised whole blood viscosity, were examined for changes in their coagulation and fibrinolytic systems. 5 were thrombocytopenic but showed no other signs of activated coagulation. Neither did the only patient with positive ethanol gelation test measuring circulating fibrin/fibrinogen degradation products (FDP) appeared in only two and, with only one exception, an assay for fibrinolytic activity in plasma was negative. No defects were found in the coagulation system. Thus, in most of the patients there was no demonstrable abnormal proteolysis in the circulation. However, in such infants the normally low levels of antithrombin III (heparin cofactor activity) in combination with the impairment of the microcirculation might increase the risk of thrombotic complications. Haemodilution, preferably with plasma, is therefore advocated in the symptomatic patients.