The records of 5 neonates with systemic arterial thrombosis (aortic in one case, peripheral in four cases) were reviewed. Fibrinolysis was performed with urokinase administered by infusion (1,000 to 4,000 U/kg/h). This treatment was combined with heparin therapy in 4 cases. Thrombosis was due to various causes: umbilical arterial catheter (1 case), disorders of supraventricular rhythm in utero (1 case), aneurysm of the ductus arteriosus with dysplastic aortic arch vessels (2 cases); one of these patients also had myocardiopathy. No cause could be found in a premature child weighing 1,300 g. The presenting symptoms of systemic arterial thrombosis are ischaemia of the extremities and suppression of peripheral pulses; heart failure with arterial hypertension is frequent. In our series the diagnosis was confirmed by doppler-ultrasonography in one case and by angiography in three cases (angiography in the left ventricule with foramen ovale, or umbilical aortography). Treatment with urokinase lasted 1.5 to 7 days. In 2 children the initial dosage had to be increased as there was no clinical improvement. Four children were completely cured; the fifth child, who had left renal thrombosis, shows slight functional impairment of the left kidney. There were no haemorrhagic complications. The fibrinolytic treatment with urokinase of systemic arterial thrombosis in the newborn is effective and has few drawbacks.