Background: We have used the Amplatzer ductal occluder for transcatheter closure of large persistently patent arterial ducts, and used our experience to assess the safety and efficacy of the device in young children and infants. Methods and patients: We used the Amplatzer ductal occluder prospectively in 43 patients with large patent arterial ducts, reviewing our experience to identify any problems or complications. Results: The procedure proved successful in 42 of the patients. We achieved complete occlusion of the duct in 33 (78.5 per cent) of the patients on the day of insertion. In 6 additional patients, complete occlusion occurred 1 week to 6 months after the procedure. Trivial leaks persisted in 2 patients, while one had a significant residual leak. Problems were encountered in 7 patients. The procedure failed in one, a device was wasted in 2, pulled through in 3, while we experienced kinking of the long Mullins sheath, being unable to retrieve the device, in one patient. Minor complications occurred in 6 patients, finding flow at a peak velocity of 2.2 metres per second in the descending aorta in 2 patients, and at 2.5 metres per second in 2 further patients, and flow at 2.5 metres per second in the pulmonary arteries of two patients. One patient experienced a major complication due to excessive bleeding. Out of the 14 patients suffering adverse events, 13 weighed less than 10 kilograms. This rate of problems and complication in these patients weighing less than 10 kilograms was significantly higher than in the patients weighing more than 10 kilograms. Conclusion: Transcatheter occlusion of moderate to large patent arterial ducts with the Amplatzer ductal occluder device is safe and effective, with a high rate of complete occlusion. Problems and minor complications may be encountered in children weighing less than 10 kilograms. If the device is to be deployed completely in the ductal ampulla, and to avoid descending aortic obstruction, the size of the retention flanges of the occluder should not exceed the largest diameter of the patent arterial duct.