INTRODUCTION: Conduction abnormalities associated with long QT syndrome (LQTS) have been reported as "pseudo 2:1 AV block" due to sinus intervals shorter than ventricular refractoriness. METHOD AND RESULTS: We report the electrophysiologic characteristics of a patient suffering from congenital LQTS with episodes of true 2:1 AV block. Induction of 2:1 infra-Hisian blocks and return to 1:1 conduction were observed using single atrial and ventricular extrastimuli. The block was located in the Purkinje network but not in the myocardium. CONCLUSION: The His-Purkinje system of our LQTS patient displayed dynamic properties with a strong increase in refractoriness for short-long sequences and a decrease for long-short sequences that triggered intermittent 2:1 AV blocks.