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. If necessary, the step balloon may be expanded to securely seat the prosthetic valve at the site of the native valve. The prosthetic valve is preferably coupled to the valve catheter by a plurality of flexible extension arms which allow the prosthetic valve to be collapsed after initial deployment such that the prosthetic valve may be repositioned if necessary.