Repeated invasive electrophysiological studies in a 7-year-old boy with the classic Jervell and Lange-Nielsen syndrome disclosed increased ventricular refractoriness, unusual late diastolic endocardial waveforms, and the inability to induce ventricular fibrillation. Despite apparently beneficial electrophysiological responses to left stellate block, surgical left cervical sympathectomy was followed by spontaneous ventricular fibrillation, an unchanged QT interval and ventricular refractoriness, and persistence of late diastolic endocardial waveforms. These electrophysiological observations, which are unique, may reflect temporal and spatial inhomogeneity of repolarisation resulting from cardiac autonomic imbalance. The absence of electrophysiological change despite left stellate block is disconcerting and re-emphasises our incomplete understanding of the pathogenesis of syndromes involving long QT intervals. Invasive electrophysiological study should be considered in the assessment of future patients with this disorder.