SummaryAutomated microanalytic chromogenic coagulation assays allow serial monitoring of critically ill newborn infants. In this study 84 premature infants [26 healthy prematures and 58 neonates with idiopathic respiratory distress syndrome (IRDS)] were studied daily during the first week of life, to investigate the possible significance of hemostatic abnormalities in IRDS.In neonates with IRDS, coagulation factors II and X, antithrombin III (AT-III), plasminogen, and α2-antiplasmin were significantly lower than control values. Recovery of the initially low AT-III levels was delayed relative to the other coagulation parameters measured. An AT-III<0.15 U/ml was present within the first 6 h of life in eight patients who developed IRDS, seven of whom died within 48 h. Autopsy of these neonates showed widespread fibrin deposition and hemorrhage in vital organs consistent with intravascular coagulation. These findings indicate that very low levels of AT-III are associated with disseminated intravascular coagulation in neonates with IRDS and suggest that a deficiency of AT-III is predictive of a poor outcome.