The pacemaker syndrome is a complex of clinical signs and symptoms related to the adverse hemodynamic and electrophysiologic consequences of ventricular pacing in the absence of other causes. Neurologic symptoms or those suggesting low cardiac output or congestive heart failure, temporally related to the onset of ventricular pacing, are indicative of the pacemaker syndrome. The evolution of a clinically recognized syndrome, an analysis of possible mechanisms and clinical manifestations, and diagnostic approaches and their implications for management are discussed.