Background: Sufficient left ventricular volume is required for patients with tetralogy of Fallot (TOF) who are going to have biventricular repair. In this study, we investigated the utility of the electrocardiogram to evaluate left ventricular volume in patients with TOF. Method: Patients whose left ventricular (LV) end-diastolic volume was lower than 80% of normal were defined as having a small LV. Seven patients with TOF who had to undergo Blalock–Taussig shunt surgery because of a small LV were assigned to group S. Twenty patients with TOF who had sufficient LV volume were assigned to group G. The amplitudes of the Q wave of V5–7 leads (QV5–QV7), the S wave of V1 lead, and the R wave of the II, III, aVf, and V5–7 leads of the electrocardiogram were evaluated. Results: The amplitude of QV5 was 0 mV in all cases in group S, which was significantly smaller than that in group G (0 vs 0.01 mV, P = 0.028). The frequency of absent QV5 was significantly higher in group S than in group G (100% vs 50%, P = 0.026). Absent QV5 showed 100% sensitivity, 50% specificity, and a negative predictive value of 100% for a small LV. Conclusions: In TOF, the amplitude of the septal Q wave reflects LV volume. In particular, the absence of QV5 suggests a small LV end-diastolic volume, which is lower than 80% of normal. [ABSTRACT FROM AUTHOR]