The occurrence of knots and loops is a potential hazard of a balloon-tipped, flow-directed pulmonary artery (PA) catheter placement if excessive catheter length is passed into the right atrium or ventricle. Knotting of a balloon-tipped, flow-directed PA catheter leading to difficulty in its removal is a rare but serious complication. A case of knotted catheter in right atrium in a patient undergoing aortic valve replacement is presented. By passing a spring guidewire into PA catheter, we have untied the loose knotted catheter under simple fluoroscopic guidance in the intensive care unit.