The case of a 13-year-old child with an unusually localized "focal" permanent atrial tachycardia is reported. Electrophysiologic study showed that the earliest atrial activation occurred in the distal coronary sinus and preceded the atrial depolarization recorded along the endocardial side of the lateral part of the mitral annulus. Distal coronary sinus mapping revealed a fragmented, polyphasic atrial electrogram. Radiofrequency current delivery to the site permanently stopped the tachycardia.