T he idiopathic long QT syndrome (LQTS) is a congenital disease with frequent familial transmission, characterized primarily by prolongation of the QT interval and by the occurrence of life-threatening tachyarrhythmias, particularly in association with emotional or physical stress. 1-5 Among untreated symptomatic patients, lethality is high, with 20% mortality in the first year after the initial syncope and approximately 50% within 10years3; however, the risk of death varies among different families. This poor prognosis has been significantly improved by the use of pharmacological or surgical antiadrenergic therapy or both, which has reduced long-term mortality to< 5%. 3, 4, 6 The availability of effective therapy for this often lethal disease emphasizes the importance of early and accurate diagnosis. Unfortunately, there is frequently a delay in the diagnosis of LQTS, and patients with syncope are often misdiagnosed, most commonly as affected by a seizure disorder. In its most characteristicpresentation, with obvious QT prolongation and stress-induced syncope, the diagnosis of LQTS is quite straightforward for physicians aware of the disease. In cases of borderline QT prolongation and/or absence of symptoms, however, a correct diagnosis may be more difficult. It was for this reason that a first set of diagnostic criteria (Table 1) was proposed in 1985.3 The major merit of that proposal was that itprovided a logical andquantitative approach to the clinical diagnosis of LQTS by giving a different weight to major and minor criteria. Its major limitation was that it used thetraditional, but untested for diagnostic purposes, cutoff value of QT,> 440 msec'2. This also resulted in a rather black-and-white situation in which patients were judged to have an entirely normal or abnormal duration of ventricular repolarization on the basis of a difference of a few milliseconds in a measurement fraught with difficulties, such as interob-server variability. 7