The purpose of this study was to evaluate changes in the left atrial size in acute mitral regurgitation (AMR) and monitor its correction by a catheter-mounted balloon (B). In 16 dogs, pressure changes in the left atrium (LA) were related to LA size and regurgitant mitral flow after the production of AMR by severing the mitral cusps. TEE was used for evaluating LA area (ALA) and the mitral regurgitation flow area (AMRF). TEE was also used in monitoring the position of a catheter-mounted B inserted through the LV apex and positioned on the mitral ring to relieve AMR. The B was inflated during systole and deflated during diastole. The ALA increased by 4.89 +/- 1.21 cm2 (mean +/- 1 SD) (p < 0.001) after AMR, the AMRF increased by 3.01 +/- 0.85 cm2 (p < 0.001) and the mean atrial pressure (LAP) by 9.38 +/- 2.43 mmHg (p < 0.001). In all experiments the position of the B could be confirmed in the 2D image and in 4 the reduction of AMRF by the function of the B was documented by the colour flow Doppler. It is concluded that after AMR the size of the LA increases along with the increase in LA pressure and a phasically inflated balloon and its effect on AMR can be easily identified and monitored by TEE.