To assess the frequency of tricuspid regurgitation (TR) and the effect of atrial contraction on the TR flow in right ventricular (RV) pacing, we examined 25 patients with RV pacing by continuous wave Doppler echocardiography. Of these patients 14 had sinus P waves, 3 retrograde P waves and 5 atrial fibrillation on the electrocardiogram. Presence or absence of notch formation of TR signal and the duration of TR on continuous wave Doppler echocardiogram were analyzed in relation to the timing of P waves. TR was detected in 22 of 25 patients (88%). The peak velocity of TR was 2.0±0.2 m/sec (Mean±SD). When P wave appeared in systole, notch formation of TR was observed in 11 of 14 patients with sinus P waves and in 1 of 3 patients with retrograde P waves, but not observed in all the patients with atrial fibrillation. In all patients who had sinus P waves, the duration of TR was significantly shortened when P wave appeared in systole than in diastole. In conclusion, TR was common in patients with right ventricular pacing and atrial contraction during ventricular systole provoked the notch formation and/or the shortening of duration of TR signal.