It is well known that some epileptic patients does not respond to conventional treatments, despite multiple combination of antiepileptic drugs, and they are therefore considered drug-resistant. For these patients, vagal nerve stimulation (VNS) represents a successful alternative to traditional therapy, and it is generally well tolerated; beside benefits on seizure frequency, VNS showed positive effects on cognition and mood. Aim of this study was to investigate short-term memory changes in a group of 12 patients implanted with VNS, through Mismatch Negativity wave (MMN). After 1 year of follow-up, MMN latencies and amplitudes did not show significant changes following VNS implantation, independently on current intensity, as compared with pre-implantation values. In two patients, MMN values, which were abnormal before VNS implantation, showed a major reduction in latency and an increase in amplitude after implantation, suggesting a likely positive effect of VNS on pre-attentive processes investigated by MMN.