A double lumen catheter made up of an elongated integral flexible plastic thin-walled tubular body extending from a proximal end part, through an intermediate part, to a distal end part, terminating in a tip. The intermediate part has an outer wall and an internal septum wall containing juxtaposed arterial and venous lumens. The terminal end of each lumen in the proximal part is adapted for connection to an access tube to hemodialysis apparatus. At the junction of the intermediate part and the distal part, the arterial lumen is terminated and the venous lumen continued at least part way to the tip of the distal part. The outside surface of the outer walls are continuous and smooth to offer minimum resistance to insertion into the body of a patient. The walls are also relatively thin to provide for lumens of relatively large cross-section compared to the total cross-section of the catheter body. Removable obturators are inserted in the lumens to prevent the catheter from buckling. The inner surface of the outer walls and the surfaces of the septum wall are continuous and smooth and any change of direction in the lumen wall is in a gradual curve to accommodate negotiation by a flexible obturator. Alternatively, the catheter may be made from tubing sufficiently resistant in itself to buckling for the catheter to be inserted without the use of obturators. One such form of tubing is made of alternating hard and soft plastic rings welded together. Another form employs wire reinforcing within the walls. Welding and molding methods for making the catheters are disclosed.