Background:Patients with hypertrophic cardiomyopathy (HCM) sometimes develop myocardial fibrosis in association with adverse cardiovascular events. Electrocardiography (ECG) could provide helpful information on myocardial fibrosis in HCM, as in coronary artery disease.Methods and Results:A total of 60 patients with HCM without bundle branch block underwent cardiac magnetic resonance imaging (CMR). The extent or location of late gadolinium enhancement (LGE) was examined in relation to 12-lead ECG. A notch on QRS was defined as at least 2 consecutive spikes in the same polarity with a reversal of direction ≥90° and the initial negative deflection ≥0.05 mV. LGE was associated with notched QRS, leftward QRS axis, and prolonged QRS duration, but not with any other findings such as abnormal Q waves, R-wave amplitude, or ST-T changes. Notched QRS was most useful in determining the presence or absence of myocardial fibrosis, with a sensitivity of 70% and a specificity of 81% using a cut-off of the number of leads with notched QRS ≥2. The number of notched QRS leads was positively correlated with LGE volume (P<0.01) and the lead distribution of notched QRS was informative about the location of LGE.Conclusions:A notch on QRS was useful in estimating myocardial fibrosis as assessed on CMR LGE in HCM patients without bundle branch block. (Circ J 2015; 79: 847–853)