['We sought to assess the efficacy of biventricular pacing with respect to both peak and submaximal measures of exercise in patients with New York Heart Association class III heart failure (HF) and intraventricular conduction delay in a randomized, blinded study.', 'Submaximal and maximal changes in exercise capacity need evaluating in this patient population with this novel therapy.', 'Graded exercise and 6-min walk tests were performed in patients randomized to three months each of active (atrio-biventricular) and inactive pacing. Minute ventilation (VE), oxygen uptake (VO(2)), ventilated carbon dioxide (VCO(2)) and heart rate were measured in patients achieving a respiratory quotient >1 (n = 30). Oxygen pulse, anaerobic threshold (AT) and ventilatory efficiency (VE/VCO(2)) were calculated.', 'Active biventricular pacing increased peak VO(2) (15.8 +/- 4.3 vs. 14.4 +/- 4.6 ml/kg/min, p = 0.02), exercise time (501 +/- 223 s vs. 437 +/- 233 s, p < 0.001) and oxygen pulse (9.3 +/- 2.8 vs. 8.1 +/- 3.1 ml/beat, p < 0.01) compared with inactive pacing. The submaximal measures of exercise capacity significantly increased with active pacing: AT (11.2 +/- 4.1 ml/kg/min vs. 9.5 +/- 2.3 ml/kg/min, p = 0.02) and 6-min walk (414 +/- 94 m vs. 359 +/- 94 m, p = 0.001). Minute ventilation/ventilated carbon dioxide improved (32 +/- 9 vs. 36 +/- 11, p = 0.03) with normalization of the VE/VCO(2) slope in 59% of patients (chi-square test, p = 0.002) with active pacing.'] Does atriobiventricular pacing improve exercise capacity in patients with heart failure and intraventricular conduction delay?