Controversy exists as to which of several ECG criteria are necessary for the diagnosis of left atrial (LA) rhythm. We performed invasive electrophysiologic study in five patients (6 to 15 years-of-age) who had symptomatic supraventricular tachycardia (SVT) that could not be controlled by aggressive pharmacologic treatment. All patients were found to have automatic atrial tachycardia with the earliest site of activation during SVT in the LA. The ECG of each patient demonstrated negative P waves in lead I. The frontal plane P wave axis ranged between +90 to +270 degrees in each of our patients. Spontaneous "dome-and-dart" P waves occurred in lead V1 in two of our patients. We conclude that the necessary criterion for the diagnosis of LA rhythm should be negative P waves in lead I. The finding of "dome-and-dart" P waves in V1 is an additional useful and definitive criterion but is not present in each case.