Sorin recently identified mode reversion to backup mode (VOO). They estimate a prevalence of 1% and suggest electromagnetic interference (EMI) as the cause. We identified ten patients being followed in our pacing clinic, of whom four were found to have suffered mode reversion. We, therefore, conclude that the prevalence of this problem may be greater than predicted. None of the patients who were found to have mode reversion gave any history to suggest that they had been exposed to EMI.