Vagus nerve stimulation (VNS) is widely used to treat refractory epilepsy. It is usually safe and has few side effects. Cardiac arrhythmia has been reported during lead tests performed during implantation of the device, but never during regular treatment. We report here a case where vagally induced bradyarrhythmia, perfectly correlated with the stimulation periods, suddenly occurred two years and four months after the VNS implantation. The diagnosis was based on the appearance of syncope-like episodes. No specific cause could be found to explain the appearance of the episodes. To our knowledge, this is the first report on this severe and life-threatening side effect of VNS and should alert clinicians to its possibility. However, considering the large number of VNS implantations performed worldwide, it must be regarded as an extremely rare complication.