Junctional ectopic tachycardia has been described in infants but not in adults. Five adults with rapid symptomatic paroxysmal junctional tachycardia, distinct from the more common slower nonparoxysmal junctional tachycardia, were recently evaluated. The tachycardia was irregular (rate 120 to 250) and accompanied by periods of atrioventricular dissociation and narrow QRS complexes. A junctional origin was documented during electrophysiologic study in four of the five patients. Analysis of Holter recordings; the response to exercise, isoproterenol, and propranolol; and the effects of atrial and ventricular stimulation appeared to implicate abnormal automaticity of a high junctional focus that was catecholamine sensitive or dependent as the tachycardia mechanism. All patients responded somewhat to β-blockers, although a combination of procainamide and propranolol proved to be the most effective therapy in one patient and another chose electrode catheter ablation of the atrioventricular junction rather than continued drug therapy. Thus, junctional ectopic tachycardia may occur in adults and its mechanism appears to be related to abnormal automaticity that is catecholamine sensitive or dependent. Initial therapy should include β-blockers but selected patients may require more aggressive management.