Since 1980, use of the transseptal approach to catheterization of the left side of the heart has increased at our institution. In our experience, the transseptal technique has been used most commonly in adult patients with aortic stenosis, in whom the ease of access and the more reliable hemodynamics make it preferable to the retrograde approach. Several recent innovations in equipment and refinement of the transseptal technique have rendered it a safer procedure than was reported in earlier experiences. Specific precautions, however, should be observed in all patients in whom the procedure is attempted and particularly in those with conditions that distort the usual anatomy of the inferior vena cava or atrial septum. Transseptal catheterization is a versatile and relatively safe procedure when it is done by experienced personnel. Because we have noted fewer complications with prolonged experience, we recommend that the procedure be restricted to high-volume cardiac catheterization laboratories that are equipped with biplane fluoroscopy.