During transseptal puncture, once the needle tip has successfully accessed the left atrium, advancement of the needle, dilator and sheath into the left atrium can risk left atrial free wall perforation, particularly if the interatrial septum is aneurysmal and tents far into the left atrial cavity during puncture. We have modified our transseptal technique such that once the left atrium is accessed with the needle tip, a 0.014″ angioplasty guidewire is advanced down the Brockenbrough needle. This is guided into the left upper pulmonary vein, and the needle, dilator and sheath advanced over this wire towards the left upper pulmonary vein. In this way, the risk of perforation of the left atrial free wall is negated. We have since used this technique in 30 cases without difficulties.