Abstract: A method for hydroforming a closed channel structural member that allows the perimeter to be increased, but which maintains a relatively uniform wall thickness throughout, includes the initial step of disposing a closed channel structural member, such as a tube, within a first hydroforming die defining a first die cavity. The inner surface of the first die cavity corresponds in cross sectional shape to the outer surface of the tube, but the perimeter of the first die cavity is somewhat larger than the perimeter of the tube enclosed therein. A preliminary hydroforming operation is then performed at a relatively high pressure to expand the tube into conformance with the first die cavity. Throughout most of this expansion, the outer surface of the tube does not engage the inner surface of the die cavity. As a result, as the perimeter of the tube is increase, the wall thickness thereof is decreased uniformly.