AIMS: The benefits of cardiac resynchronization therapy (CRT) on functional status, left ventricular (LV) remodelling and survival in patients with drug-refractory congestive heart failure (CHF), LV systolic dysfunction, and wide QRS have been demonstrated in randomized trials. However, the impact of CRT on right ventricular (RV) function, an independent prognostic factor in CHF remains questionable. This study examined the acute effects of various pacing modes on RV function in recipients of CRT systems. METHODS AND RESULTS: Echocardiographic examinations were performed in 15 patients (median age: 67 years, range 49-78), to compare RV function during atrial (AAI), RV and LV pacing, and biventricular (BiV) pacing, in random order. At baseline, the median LV ejection fraction was 20% (range 10-35) and the median LV end-diastolic diameter was 78 mm (range 62-85). Right ventricular function was impaired, with a median 36% fractional shortening of RV surfaces (7-59). Tissue Doppler systolic peak of velocity (Sa) recorded at the tricuspid annulus increased significantly from 9.9 cm/s (range 4.7-16.5) during AAI pacing, 10 cm/s (range 5.4-20.3) during RV pacing, and 11.7 cm/s (range 4.6-16.7) during LV pacing to 12.6 cm/s (range 6.6-19.1) during BiV pacing (P < 0.01). Trends toward improvements in other indices of RV function, particularly myocardial performance index and systolic excursion of the tricuspid annulus, were also observed. CONCLUSIONS: This short-term study showed a significant improvement in RV systolic function during BiV pacing compared with AAI, RV, or LV pacing in CRT recipients.