Abstract Background Voltage signals in the coronary sinus (CS) have been associated with the presence of left atrial fibrosis. The aim of the present study was to evaluate the value of CS voltage signals as a predictor for atrial fibrillation (AF)-recurrence-free outcome of pulmonary vein isolationprocedures (PVI) in patients after a first unsuccessful cryo-balloon PVI. Method We collected recordings from a diagnostic catheter positioned in the CS from 282 consecutive atrial fibrillation patients undergoing a re-dopulmonary vein isolation using a 3D mapping system. The patients were followed-up (Holter ECG and telephone calls) for at least one year (median of 14 months). Results Of the 282 patients (male 72%, mean age 63±10.8 years, 61% persistent AF) AF recurrences were documented in 152 pts (54%)with a signal amplitude in the proximal CS position of 2.4 mV ± 1.5 mV. Patients free of AF-recurrence showed significantly higher signal amplitude of 2.9 mV ± 2.1 mV (P<0,05). A CS voltage <0.53mV could predict recurrences of AF with a sensitivity of 94.7% (95% CI 89.3% – 97.8%) and specificity of 8.6% (95% CI 4.6% – 14.8%; PLR 1.04; AUC 0.55). Conclusion Voltage signals in the CS, as a marker for left atrial fibrosis, are associated with the outcome of PVI. A voltage threshold of <0.53mV can predict AF recurrences with a high sensitivity. However, the predictive value for AF recurrences is not high due to the low specificity of this test. Funding Acknowledgement Type of funding sources: None.