Cardiac resynchronization therapy (CRT) has proven prognostic benefits in patients with heart failure (HF) with left bundle branch block (LBBB) QRS morphology. Electrocardiographic left atrial (LA) abnormality has been proposed as a noninvasive marker of atrial remodeling. We aimed to assess the impact of electrocardiographic LA abnormality for prognosis in patients with HF treated with CRT. Baseline resting 12-lead electrocardiograms recorded from 941 patients enrolled in the CRT arm of the Multicenter Automatic Defibrillator Implantation Trial with Cardiac Resynchronization Therapy was processed automatically using Glasgow algorithm, which included automated assessment of P-wave terminal force in lead V₁ (PTF-V₁) as a marker of LA abnormality. A PTF-V₁ of ≥0.04 mm⋅s was considered abnormal. The primary end point was HF event and/or death. Total mortality and appropriate defibrillator therapies were the secondary end points. At baseline 550, patients treated with CRT with a defibrillator had LBBB QRS morphology and normal PTF-V₁. Normal PTF-V₁ was associated with significant risk reduction for all assessed end points and for the primary end point comprised a hazard ratio of 0.55 (95% confidence interval 0.36 to 0.84) compared with patients with LBBB with abnormal PTF-V₁ (n = 120), and a hazard ratio of 0.42 (95% confidence interval 0.32 to 0.55) compared with patients with implanted defibrillator (n = 729). In CRT-treated patients with HF, electrocardiographic LA abnormality appears to be an electrocardiographic indicator of poor long-term outcome in patients with LBBB. In conclusion, our data suggest that PTF-V₁ bears additive prognostic information in the context of CRT, thus further strengthening the role of electrocardiographic diagnostics in risk stratification of patients with HF.