Cryoablation Within the Coronary Sinus. Radiofrequency ablation inside the coronary sinus (CS) is associated with the risk of vein stenosis, or perforation or damage to the circumflex artery. Cryothermic ablation has proved less harmful, but there are no data on its use within a venous structure. A patient with a posterolateral accessory pathway underwent several unsuccessful attempts at endocardial ablation. Ablation then was performed with cool energy within the CS, where the shortest VA interval and a possible Kent potential were recorded. Selective left coronary angiography showed a normal circumflex artery and the absence of lesions of the CS. Six months later, the patient was asymptomatic without taking antiarrhythmic drugs. We conclude that cryoablation within the CS may be effective and safe.