Background: The corrected QT interval (QTc) according to Bazett's formula (QTc = QT/RR^1/2) has been used in clinical practice. Bazett's formula, however, overcorrects the QT interval at fast heart rates and undercorrects it at low heart rates. Guidelines and some investigators have recommended using Fridericia's formula (QTc = QT/RR^1/3) in these cases, especially in tachycardic subjects. The aim of the present study was to determine cut-offs for QTc suitable for screening pediatric subjects with prolonged QT intervals, based on manually measured values corrected by Fridericia's formula in a large number of subjects. Methods and Results: Three consecutive QT and RR intervals were measured in 4,655, 4,655, and 5,273 1st, 7th, and 10th graders, aged 6, 12, and 15 years, respectively. Each QT interval was corrected by Fridericia's formula, and mean values were calculated. Determination of the cut-offs for screening was based on the prevalence of abnormal electrocardiographic phenotypes of 1:1,164 and on the upper 0.025 percentile in the QTc distribution derived from previous studies. The tentative cut-offs suitable for screening subjects with prolonged QT intervals were 430 ms for 1st graders, 445 ms for 7th graders, and 440 and 455 ms for 10th grade boys and girls, respectively. Conclusions: These tentative cut-offs can be used to screen subjects with prolonged QT intervals in the clinical setting. Further studies are needed to confirm their validity.  (Circ J 2010; 74: 1663 - 1669)