OBJECTIVE: Atrial cryolesions usually are created from the endocardium with the heart arrested. Some cardiac surgeons have used cryoablation epicardially during cardiopulmonary bypass, which is convenient because it does not require an incision into the atrial wall. We analyzed the transmurality of epicardial cryoablations created during cardiopulmonary bypass in an arrested heart 30 days after ablation. METHODS: In Yucatan minipigs (n = 5), hearts were exposed via sternotomy. Both caval veins were cannulated to collect blood for the cardiopulmonary bypass. Cryolesions were created applying a cryoprobe for 4 minutes per lesion. Hearts were harvested 30 days after the surgery. The transmurality of each lesion was determined for 3 transversal sections using triphenyl tetrazolium chloride staining. The structure of the lesions was assessed with Gomori trichrome stains. RESULTS: We created a total of 20 atrial lesions and stained 175 sections. Transmurality was achieved in 96% of the sections and 75% of lesions. A significant fraction of the sections (12%) was transmural but had a width of less than 1.5 mm. CONCLUSIONS: Although epicardial cryoablation during bypass achieved better results than epicardial cryoablation of beating hearts, it falls short of conventional cryoablation and radiofrequency ablation.