Left ventricular outflow tract (LVOT) obstruction is a complication of Carpentier ring mitral valvuloplasty that may occur only when this procedure is used to correct mitral regurgitation attributable to myxomatous degeneration of the mitral valve. LVOT obstruction has not been observed among approximately 300 patients undergoing this procedure to correct mitral regurgitation attributable to other causes. Among 200 patients with degenerative mitral regurgitation who underwent Carpentier valvuloplasty, LVOT obstruction was found in 12 patients (6%). Five of these patients demonstrated severe LVOT obstruction during intraoperative echocardiography immediately after repair, which was corrected by mitral valve replacement in four and removal of the Carpentier ring in one. The remaining seven patients were followed-up for a mean period of 27 months with history, physical examination, and Doppler echocardiography. Systolic anterior motion of the mitral valve was the echocardiographic hallmark of LVOT obstruction. Doppler echocardiographic and catheter-measured LVOT gradient paralleled the severity of the systolic anterior motion of the mitral valve. The severity of the motion decreased, but still could be provoked, with amyl nitrite at late follow-up. Mitral regurgitation tended to recur at late follow-up. Despite the presence of LVOT obstruction and hemodynamic features resembling hypertrophic cardiomyopathy at late follow-up, none of the patients had left ventricular hypertrophy or asymmetric septal hypertrophy, and early postoperative functional class improvement was sustained.