Catheter occlusion of the persistent ductus arteriosus recently became clinically available with a Rashkind (USCI Bellerica, Massachusetts, USA) umbrella occluder system. A few patients, however, had residual left to right shunting after the procedure. The impact of soaking the foam of the device in a thrombin solution to enhance clotting and of using a device with a diameter of 12 or 17 mm was assessed in 117 consecutive patients undergoing catheter closure. The frequency of residual shunting detected by colour flow Doppler 24 hours after the procedure was the same whether or not thrombin was used.