Short QT syndrome (QTc ? 300 ms) is a novel hereditary channelopathy linked to syncope, paroxysmal atrial fibrillation, and sudden cardiac death. However, its epidemiological features remain unsettled. (1) To assess the prevalence of short QT in a large population-based sample; (2) to evaluate its demographic and clinical correlates and; (3) to determine its prognosis. A database of 6.4 million electrocardiograms (ECGs) obtained between 1995 and 2008 among 1.7 million persons was used. An internal, population-based method for heart rate correction (QTcreg ) was used and all ECGs with QTcreg ?300 ms were manually validated. Linked health plan databases were used for covariate and survival ascertainment. Of 6,387,070 ECGs, 1086 had an ECG with machine-read QTcreg ?300 ms. Only 4% (45/1086) were validated yielding a prevalence of 0.7 per 100,000 or 1 of 141,935 ECGs. At the person level, the overall prevalence of QTcreg ?300 ms was 2.7 per 100,000 or 1 of 37,335. The factors independently and significantly associated with validated QTcreg ?300 ms were age over 65 years, Black race, prior history of ventricular dysrhythmias, chronic obstructive pulmonary disease, ST-T abnormalities, ischemia, bigeminy pattern, and digitalis effect. After 8.3 years of median follow-up and relative to normal QTcreg , validated QTcreg ?300 ms was associated after multivariate adjustment with a 2.6-fold (95% confidence interval [CI] = 1.9-3.7) increased risk of death. QTcreg ?300 ms was extraordinarily rare and was associated with significant ECG abnormalities and reduced survival.