Abstract: A heart valve delivery system is provided wherein a prosthetic valve is carried on a valve catheter inside a delivery sleeve. A step balloon protrudes from the delivery sleeve and provides a tapered surface for facilitating advancement through a body vessel. The step balloon also aids in crossing the leaflets of a native valve. After the prosthetic valve is positioned within the native valve, the delivery sleeve is retracted to expose the prosthetic valve. In one embodiment, the delivery sleeve is retracted by the use of a lead screw, which effectuates relative movement between the valve catheter and delivery sleeve. The prosthetic valve is preferably self-expandable. If necessary, the step balloon may be expanded to securely seat the prosthetic valve at the site of the native valve.