In the manufacture of a tubular member which is to be concave inward as viewed in longitudinal section and precisely symmetrical on opposite sides of its transverse centerline, such as a hi-pass millimeter waveguide filter, a tubular billet initially is fabricated with temporary end flanges having circumferential grooves for receiving hydrostatic seals during a stress-forming operation. Next, the tubular billet is positioned in a hydrostatic forming device and a split mandrel is positioned within the billet and retained against movement. Hydrostatic pressure then is applied to the tubular billet to stress the billet longitudinally in tension while compressively stressing the billet inward into formed engagement with the mandrel, to reduce the hydrostatic pressure required in the stress-forming operation and to achieve the desired symmetry in the formed billet. The hydrostatic stress-forming of the billet preferably is accomplished in successive steps, with intermediate annealing, using mandrels of progressively decreasing size, and the billet may be stretched longitudinally while being formed inward. When the hydrostatic stress-forming of the billet has been completed, portions of the temporary end flanges which define the seal receiving grooves are removed from each end flange and the end flanges are formed to a final desired configuration.