['Renal dysfunction is an important independent prognostic factor in heart failure (HF). Cardiac resynchronization therapy (CRT) improves functional status and left ventricular (LV) function in HF patients with ventricular dyssynchrony, but the impact of CRT on renal function is less defined. We hypothesized that CRT would improve glomerular filtration rate as estimated by the abbreviated Modification of Diet in Renal Disease equation (eGFR).', 'The Multicenter InSync Randomized Clinical Evaluation (MIRACLE) study evaluated CRT in HF patients with NYHA Class III-IV, ejection fraction <or=35%, and QRS >or=130 ms. Patients were evaluated before and 6 months after randomization to control (n = 225) or CRT (n = 228). Patients were categorized according to their baseline eGFR: >or=90 (category A), 60 <or=eGFR <90 (category B), and 30 <or=eGFR <60 (category C) mL/min per 1.73 m(2). CRT improved LV function in all categories. Compared with control, CRT increased eGFR (-2.4 +/- 1.2 vs. +2.7 +/- 1.2 mL/min per 1.73 m(2); P = .003) and reduced blood urea nitrogen (+6.4 +/- 2.4 vs. -1.1 +/- 1.5 mg/mL; P = .008) in category C, whereas no differences were observed in categories A and B.'] Does cardiac resynchronization therapy improve renal function in human heart failure with reduced glomerular filtration rate?