Percutaneous radiofrequency catheter ablation has been recently introduced for treatment of Wolff-Parkinson-White syndrome. Access to left free-wall atrioventricular accessory pathways can be obtained either via retrograde cardiac catheterization or via the transseptal procedure, which allows ablation of the accessory pathway at its ventricular or atrial insertion, respectively. We describe a patient with Wolff-Parkinson-White syndrome in whom coronary air embolism occurred as a complication of transseptal percutaneous radiofrequency catheter ablation. The diagnosis was made by two-dimensional echocardiography showing a marked echocontrast effect in the posterior wall and in the posterior half of the interventricular septum. A grossly evident breakage of the rubber seal of the vascular sheath was supposed to be the cause of air insinuation. This report suggests that the transseptal approach should be used with caution in performing percutaneous radiofrequency catheter ablation to avoid the risk of air embolization. Two-dimensional echocardiography is an ideal tool to detect this complication.