Background - Cryoablation procedures for pulmonary veins isolation have proved to be a successful treatment of atrial fibrillation, but exposure of surrounding organs to excessively low temperatures is potentially dangerous. Hence the importance of monitoring esophageal temperature and at the same time predicting the thermal field induced by the procedure to prevent the onset of the lesions. . Methods: We formulated a mathematical model for computing the esophageal temperature and we used numerical simulations to interpret recorded clinical data. Clinical data have been collected during cryoballon isolation of pulmonary veins in patients with atrial fibrillation. Cryo-ablation procedures were performed in 9 patients (36 PV) with a cryo-balloon Arctic Front Advance (Medtronic, Inc, Minneapolis, MN, USA). Luminal esophageal temperature (LET) was recorded by means of the Esotherm catheter (FIAB SpA, Vicchio, Italy). Results - The temperature at the outer esophageal surface resulted much lower than the luminal one (p<0.001) with a difference that in some cases was as large as 16° C. The cooling rate of LET at the early stage was related to the achievement of lower temperatures later during cryo-energy application, with an identified dangerous threshold of 0.15° C/sec. The steepness of the esophageal thermal gradient was related to the achievement of dangerous values even if LET was far from an alarm threshold. Three of the nine patients were subjected to a marked esophageal cooling and the clinical data during cryo-energy application are compatible with the mathematical model. Conclusion - Monitoring the time evolution of luminal esophageal temperature is of fundamental importance not only to record but also to predict well in advance critical developments of the temperature drop. LET is higher than the temperature recorded on the external esophageal surface. The model predictions fit remarkably well the data recorded during clinical cryoballon ablation procedures.