Classic electrocardiographic (ECG) voltage indexes have been applied to screen for left ventricular (LV) hypertrophy in hypertrophic cardiomyopathy (HC). However, it is unclear whether low ECG voltage reflects deteriorated electrical forces because of replacement of the myocardium by fibrotic tissues in HC. We investigated correlations between classic ECG voltage indexes (Cornell, total QRS voltage, and Sokolow-Lyon) and cardiac magnetic resonance (CMR) parameters focusing on the impact of low ECG voltage on the LV ejection fraction (LVEF) and myocardial fibrosis in HC. We studied 108 consecutive patients with HC who underwent CMR imaging with late gadolinium enhancement (LGE). Nineteen patients with complete right or left bundle branch block were excluded, leaving 89 patients for analysis (age 61.0 ± 13.9 years; 58 men). Of the 3 voltage indexes, the total QRS voltage and Sokolow-Lyon indexes were positively correlated with LVEF. For discriminating patients with end-stage HC (LVEF <50%) from patients with HC and preserved LVEF (≥50%), receiver-operating characteristic analysis revealed an excellent area under the curve of 0.87 for the total QRS voltage index and 0.90 for the Sokolow-Lyon index, whereas the area under the curve for the Cornell index was only 0.54 (p <0.01). Moreover, these 2 voltage indexes were negatively correlated with the extent of LGE-determined myocardial fibrosis when adjusted by the LV maximal wall thickness. In conclusion, low ECG voltage indexes may reflect increased myocardial fibrosis in patients with HC.