OBJECTIVES We sought to investigate the impact of six months of cardiac resynchronization therapy (CRT) on echocardiographic variables of left ventricular (LV) function. BACKGROUND Cardiac resynchronization therapy has recently been introduced as a new therapeutic modality in patients with advanced heart failure (HF) and conduction abnormalities. However, most studies have only investigated the early hemodynamic effects of CRT. METHODS Twenty-five patients (12 women and 13 men; 59.8 5.1 years old) with advanced HF caused by ischemic (n 7) or idiopathic dilated cardiomyopathy (n 18) and a prolonged QRS complex were analyzed. All patients underwent early hemodynamic testing with a randomized testing protocol; echocardiographic measurements were compared before implantation and after six months of CRT. RESULTS Left ventricular end-diastolic and end-systolic diameters (LVEDD and LVESD, respectively) were significantly reduced after six months (LVEDD from 71 10 to 68 11 mm, p 0.027; LVESD from 63 11 to 58 11 mm, p 0.007), as were LV end-diastolic and end-systolic volumes (LVEDV from 253 83 to 227 112 ml, p 0.017; LVESV from 202 79 to 174 101 ml, p 0.009). Ejection fraction was significantly increased (from 22 7% to 26 9%, p 0.03). “Nonresponders,” with regard to LV volume reduction, had significantly higher baseline LVEDV, compared with “responders” (351 52 vs. 234 74 ml, p 0.018). Overall, there was only mild mitral regurgitation at baseline, with a minor reduction by semiquantitative analysis. The results of early hemodynamic testing did not predict the volume response. CONCLUSIONS Cardiac resynchronization therapy may lead to a reduction in LV volumes in patients with advanced HF and conduction disturbances. Volume nonresponders have significantly higher baseline LVEDV. (J Am Coll Cardiol 2001;38:1957– 65) © 2001 by the American College of Cardiology