In seven patients with Romano-Ward syndrome, stellate ganglion block or stimulation and pharmacologic interventions were made to assess their influence on duration of the O-T interval, electrical alteration of the T wave and ventricular tachydysrhythmias. Left stellate ganglion block and right stellate ganglion stimulation shortened Q-T interval, abolished alternans phenomena and suppressed tachydysrhythmias. Propranolol and phenytoin had a similar effect. In contrast, right stellate ganglion block, left stellate ganglion stimulation and prior administration of quinidine and procainamide had an opposite effect. These responses resemble observations in animal models which suggest that excessive or unopposed activity of the left, or subnormal activity of the right stellate ganglion, or both, account for the pathophysiologic manifestations of the long Q-T syndrome. They are also consistent with clinically correlated, cardiac neuropathologic findings in these patients. An analogous but acquired dysautonomia involving the left stellate ganglion and ischemic left ventricle may precipitate sudden coronary death.