A multi-lumen lay-flat tube formed of polymeric film material, e.g., polyurethane elastomer film, comprising a main tube and at least one interior tubular passage, wherein each of the at least one interior tubular passage is constituted by reentrant portions of polymeric film material forming the main tube. The multi-lumen lay-flat tube can be utilized for fabrication of a balloon catheter, or other indwelling catheter device. The multi-lumen lay-flat tube can be formed by a fabrication method including provision of superposed sheets of polymeric film material, welding the superposed sheets of polymeric film material along generally parallel, spaced-apart seam lines to form at least two passages bounded by a successive pair of such seam lines, and if desired everting the welded superposed sheets to form a main tube having at least one tubular passage therewithin. In another implementation, a multi-lumen lay-flat tube can be formed by bonding concentric tubes to one another along longitudinally extending seals, and in a still further implementation, a multi-lumen lay-flat tube can be formed by bonding a strip member longitudinally along a tube, with the strip member being bonded at its margins to the tube, to enclose a passage between the strip member and the tube.
