The case of a one-year-old child in whom a large catheter-related right atrial thrombus of more than a week's duration was dissolved by urokinase is presented. After one week of unsuccessful heparin treatment, urokinase, via a central venous catheter, was added at a dose of 1500 IU/kg/h. Urokinase was subsequently increased by 1000 U/kg/h every day up to a maximum of 4500 IU/kg/h on the basis of thrombus size reduction as assessed by daily cross-sectional echocardiography. One week later the thrombus was almost completely dissolved. No major bleeding occurred. Warfarin was given for the next three months and the international normalized ratio (INR) was maintanied between 2 and 3. At the end of warfarin treatment echocardiography was negative for right atrial thrombosis. This case suggests that local high dose urokinase therapy can be safely and successfully used to dissolve large right atrial thromboses of more than a week's duration.