Cardiac resynchronization therapy (CRT) is an established therapy for patients with severe heart failure, depressed left ventricular function and a wide QRS-complex. Large clinical trials have demonstrated unequivocal improvements in functional status, morbidity and mortality. However, 30 % of heart failure patients treated with a CRT-device do not benefit clinically. Several factors have been suggested to be important for the response to CRT such as mechanical dyssynchrony, presence of scar tissue in the myocardium, and device-optimization (among others). It is the purpose of this study to investigate the importance of these factors.