The invention comprises retaining apparatus for transseptal catheterization and a procedure for the use of same. The retaining apparatus is a selectively deployable retaining means disposed at or proximal to the tip of a transseptal catheter or sheath which is placed across a septum, such as the interatrial septum, into the left atrium. The retaining means is configured to cooperate with the distal portion of the sheath to present a uniform cylindrical circumferential periphery when the retaining means is undeployed and to present a shape generally transverse to the longitudinal axis of the sheath when deployed. In a preferred embodiment, the retaining means is a retaining balloon which, upon inflation, presents a relatively flat, disk-like shape to comprise a physical barrier preventing inadvertent retraction of the distal tip of the sheath from the left atrium during subsequent portions of the catheterization procedure. The transseptal catheterization procedure includes the steps of placing the distal tip of the transseptal sheath across a septum, deploying the retaining means for retaining the distal tip within the left atrium, completing the transseptal portion of the catheterization procedure, retracting the selectively deployable retaining means, withdrawing the distal tip from the septum and completing the procedure.