To assess the hemodynamic factors affecting left-to-right shunting following balloon mitral valvuloplasty we studied 10 patients at 3 points during the procedure: pre-septal dilation, post-septal dilation pre-valvuloplasty, and post-valvuloplasty. Hemodynamic parameters were recorded and an oximetry run was obtained at each stage. Mitral valvuloplasty was successful in 8 and unsuccessful in 2 patients. No patient had a significant oxygen step-up at any stage, including the 2 patients with unsuccessful valvuloplasty. Significant left-to-right shunting through the atrial septal puncture in mitral valvuloplasty is infrequent, even before successful balloon valvuloplasty when left atrial pressure is maximally elevated.