A delivery system for delivering a prosthetic aortic heart valve to a patient's native aortic valve, comprising: a balloon catheter comprising an elongated shaft and an inflatable balloon mounted at a distal end of the elongated shaft, and a flex indicating device comprising: a guide catheter comprising a handle portion and an elongated guide tube extending distally from the handle portion, the elongated shaft of the balloon catheter extending coaxially through the elongated guide tube; at least one pull wire connected to a distal end portion of the elongated guide tube; and a slide member connected to the at least one pull wire; wherein the handle portion comprises a flex activating member, the flex activating member being coupled to the at least one pull wire such that manual adjustment of the flex activating member causes the distal end portion of the elongated guide tube to flex; the flex indicating device further comprising a flex indicating member having an extending portion, wherein manual adjustment of the flex activating member causes the flex indicating member to move relative to the handle portion to indicate an amount of flex of the distal end portion of the elongated guide tube wherein the flex activating member comprises a rotatable member (155, 157) which includes an internally threaded surface portion and an externally threaded surface portion, wherein the internally threaded surface portion is configured to receive the slide member connected to the at least one pull wire, and the externally threaded surface portion is configured to receive the extending portion of the flex indicating member. a balloon catheter comprising an elongated shaft and an inflatable balloon mounted at a distal end of the elongated shaft, and a flex indicating device comprising: a guide catheter comprising a handle portion and an elongated guide tube extending distally from the handle portion, the elongated shaft of the balloon catheter extending coaxially through the elongated guide tube; at least one pull wire connected to a distal end portion of the elongated guide tube; and a slide member connected to the at least one pull wire; wherein the handle portion comprises a flex activating member, the flex activating member being coupled to the at least one pull wire such that manual adjustment of the flex activating member causes the distal end portion of the elongated guide tube to flex; the flex indicating device further comprising a flex indicating member having an extending portion, wherein manual adjustment of the flex activating member causes the flex indicating member to move relative to the handle portion to indicate an amount of flex of the distal end portion of the elongated guide tube wherein the flex activating member comprises a rotatable member (155, 157) which includes an internally threaded surface portion and an externally threaded surface portion, wherein the internally threaded surface portion is configured to receive the slide member connected to the at least one pull wire, and the externally threaded surface portion is configured to receive the extending portion of the flex indicating member. a guide catheter comprising a handle portion and an elongated guide tube extending distally from the handle portion, the elongated shaft of the balloon catheter extending coaxially through the elongated guide tube; at least one pull wire connected to a distal end portion of the elongated guide tube; and a slide member connected to the at least one pull wire; wherein the handle portion comprises a flex activating member, the flex activating member being coupled to the at least one pull wire such that manual adjustment of the flex activating member causes the distal end portion of the elongated guide tube to flex; the flex indicating device further comprising a flex indicating member having an extending portion, wherein manual adjustment of the flex activating member causes the flex indicating member to move relative to the handle portion to indicate an amount of flex of the distal end portion of the elongated guide tube wherein the flex activating member comprises a rotatable member (155, 157) which includes an internally threaded surface portion and an externally threaded surface portion, wherein the internally threaded surface portion is configured to receive the slide member connected to the at least one pull wire, and the externally threaded surface portion is configured to receive the extending portion of the flex indicating member.