A 9-year-old boy, with significant left-to-right shunting across a large duct in the context of rubella syndrome, was tested during catheterization to establish the feasability of occluding the duct with a device. The testing, including temporary closure of the duct and monitoring of pulmonary vascular reactivity to vasodilative substances, lead to the decision to implant an Amplatzer occluder. Sixteen months later, there was no residual shunting across the duct, and pulmonary arterial pressures had normalised. It remains unclear why the patient had not developed irreversible pulmonary vascular disease.