['The prognostic implications of low QRS voltage on the electrocardiogram (ECG) in heart failure (HF) are not well characterized.', 'We manually measured and summed the QRS voltage in all 12 leads of the ECG (sumQRS) in two cohorts: (1) 415 patients with a low left ventricular ejection fraction followed up in a HF clinic ("clinic cohort") and (2) 100 subjects with advanced HF who had an ECG within 1 year preceding cardiac transplantation ("pretransplant cohort"). Low voltage was defined as the lowest quartile of the clinic cohort (sumQRS <12 mV) and its prevalence was compared in the two cohorts. The associations of low voltage with 1-year outcomes were assessed in the clinic cohort.', 'In the clinic cohort, the frequency of low voltage was higher in New York Heart Association class 4 versus class 1-3 patients (34% vs 22% respectively, P = .04). The frequency of low voltage in the pretransplant cohort (47%) was twice that of the clinic cohort (24%, P < .001). After 1 year of follow-up in the clinic cohort, low ECG voltage was associated with a higher rate of death (14% vs 5%, P = .008) and the composite end point of death or HF hospitalization (35% vs 20%, P = .004). These associations persisted in multivariable analyses adjusting for important confounders.'] Is low voltage on the electrocardiogram a marker of disease severity and a risk factor for adverse outcomes in patients with heart failure due to systolic dysfunction?