METHOD: A prospective cohort study of 31 consecutive newborn infants with UAC-associated aortic thrombi which were detected by abdominal ultrasonography after removal of UAC. Twenty-two infants were treated with intravenous infusion of low dose (1000 U/h) streptokinase, while nine others were not treated due to various contra-indications. Thrombolysis occurred after a mean interval of 2.2 days (standard deviation (SD) = 1.8) in the treated infants. In the untreated infants, spontaneous thrombolysis occurred significantly later, after a mean interval of 16.9 days (SD = 14.7) (95% confidence intervals of difference between mean intervals - 26.0, - 3.4; P = 0.02). Only one treated infant developed mild bleeding directly attributed to streptokinase therapy.