Delivery apparatus and methods for delivering a medical device into a patient's vasculature are disclosed. In some examples, a method includes inserting, into a patient, a distal end portion of a delivery apparatus including a radially compressed prosthetic valve mounted around a balloon, advancing the distal end through the patient's vasculature, and inflating the balloon to radially expand the prosthetic valve at a native heart valve. The delivery apparatus includes a handle, a first shaft and a second shaft extending from the handle, and a locking mechanism to prevent axial movement of the second shaft relative to first shaft. The second shaft extends coaxially through the first shaft and includes the balloon, the radially compressed prosthetic valve, and one or more radiopaque markers at a distal end portion. The first shaft includes an end piece covering a proximal portion of the balloon during the advancing of the delivery apparatus.