Left bundle branch block (LBBB) is known to impair the mechanical function of the left ventricle. For a better understanding of the impact of LBBB on the structure and function of the heart and how this influences the management and outcome in patients with heart failure, we reviewed the most pertinent articles published from 1950 to 2007. Epidemiological studies identified LBBB as an independent risk factor for cardiac mortality. It is not only the morphology or duration of QRS but also the underlying myocardial pathology, ejection fraction or New York Heart Association class that may determine the clinical implications of LBBB. Data indicate that LBBB inversely affects the perfusion, systolic, diastolic performance and hemodynamics of the heart. However, there is no solid evidence answering the question of whether LBBB is a predictor, cause or consequence of myocardial dysfunction. Further studies are warranted for better understanding of these relationships, as this will allow better selection of patients and optimal time for resynchronization therapy in patients with LBBB.