At a recent meeting of the British Cardiac Society concern was expressed at the risks of piercing the aorta during attempted transseptal catheterization of the left atrium. We wish to describe a needle designed to overcome this risk, which we have used at the Postgraduate Medical School of London for the past three years.Catheterization ofthe left side ofthe heart from the venous route by means of needle puncture of the interatrial septum was introduced in 1959 (Ross, 1959;Cope, 1959). The needle that was designed for this purpose had a wide bore (17 gauge) and allowed the passage of PE 50 polythene tubing through the needle and on into the left atrium and ventricle. The technique was improved by Brockenbrough and Braunwald (1960) who designed a needle that allowed larger catheters to be introduced into the left side of the heart over the needle. The advantages of this system were that the catheter and needle could be introduced by the percutaneous route via the femoral vein, the larger catheter allowed more accurate recordings of pressures, and selective angiocardiography could be carried out through it. Since this needle was employed the technique has come into widespread use as a method of major importance (Braunwald et al., 1962;Miller and Medd, 1964).However, transseptal catheterization has been associated with complications caused generally by inadvertent puncture either of the free wall of the atrium or of the aorta which is closely related to the anterior part of the interatrial septum. These complications have, in most cases, occurred in the hands of occasional or inexperienced operators but have also been more frequent since the Brockenbrough needle and catheter replaced the original