Background: QRS duration and morphology including left bundle branch block (LBBB) are the most widely used electrocardiogram (ECG) markers for assessing ventricular dyssynchrony and predicting heart failure (HF). However, the vectorcardiographic QRS area may more accurately identify delayed left ventricular activation and HF development. Objective: We investigated the association between QRS area and incident HF risk in patients with LBBB. Methods: By crosslinking data from Danish nationwide registries, we identified patients with a first-time digital LBBB ECG between 2001 and 2015. The vectorcardiographic QRS area was derived from a 12‑lead ECG using the Kors transformation method and grouped into quartiles. The endpoint was a composite of HF diagnosis, filled prescriptions for loop diuretics, or death from HF. Cause-specific multivariable Cox regression was used to compute hazard ratios(HR) with 95% confidence intervals(CI). Results: We included 3316 patients with LBBB free from prior HF-related events (median age, 72 years; male, 40%). QRS area quartiles comprised Q1, 36–98 μVs; Q2, 99–119 μVs; Q3, 120–145 μVs; and Q4, 146–295 μVs. During a 5-year follow-up, 31% of patients reached the composite endpoint, with a rate of 39% in the highest quartile Q4. A QRS area in quartile Q4 was associated with increased hazard of the composite endpoint (HR:1.48, 95%CI:1.22–1.80) compared with Q1. Conclusions: Among primary care patients with newly discovered LBBB, a large vectorcardiographic QRS area (146–295 μVs) was associated with an increased risk of incident HF diagnosis, filling prescriptions for loop diuretics, or dying from HF within 5-years.