A new type of guiding catheter that may be used for retrograde transmitral catheterization of the left atrium is described. The catheter has a 9F diameter and incorporates a steering arm by means of which the catheter tip may be made to curve through an arc of 0 to 180 degrees, entirely through external manipulation. In this way the tip of the guiding catheter may be positioned below the mitral valve orifice and used for the introduction of a smaller catheter into the left atrial cavity. We tested the system in 20 patients, eight of whom had mitral stenosis. Left atrial catheterization was successful in all cases, and no difficulties were encountered in entry of the left atrium, nor were there any complications during or after the procedure. The technique will provide useful in the study of left atrial function (hemodynamic, electrophysiologic, secretory). It may also be helpful in percutaneous mitral valvuloplasty by permitting retrograde, rather than transseptal, access to the left atrium.