The QT interval extends from the beginning of the QRS complex to the end of the T wave. Since the report of Jervell and Lange-Nielsen in 1957, it has been difficult to determine whether to measure a QT or a QU interval. U waves are more prominent in the left chest leads and less prominent in lead II, with the end of the T wave in lead II corresponding to the end of significant repolarization in any other lead. Therefore, by convention, lead II has been chosen to measure the QT interval. Numerous attempts at correction have been made, but Bazett's correction (QT/square root of RR) remains the current standard. In stable sinus rhythm, a QT interval corrected for heart rate (QTc) of > 0.44 sec is considered abnormal. In this study, in the presence of sinus arrhythmia, the QT interval following the shortest RR interval was > 0.46 sec in 98.4% of patients with the congenital long QT syndrome, but in only 3.8% of control subjects (p < 0.0001). Likewise, in the presence of sinus arrhythmia, the uncorrected QT interval varied by > 0.03 seconds in a 10-sec rhythm strip of lead II in 33% of long QT syndrome patients but in 0% of controls (p < 0.01). The QT interval varied depending on autonomic tone and state of wakefulness, being approximately 19 msec longer in sleeping patients with a heart rate of 60 beats/min than in awake patients with the same heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)