A stretchable-diameter inserter (1), for hemodynamic studies and related operations, is characterized in that it comprises an inserter tube (2), a hemostatic valve (3) designed for preventing blood from exiting the patient toward the operator, a side line (4) including a faucet, adapted to be used for performing blood taking and infusion operations and two dilatators: a first dilatator (66) having a smaller cross-section and a second dilatator (6) having a larger cross-section. The inserter tube is made of a rigid and flexible material with a substantially circular or oval contour, and comprises one or more longitudinally extending reduced thickness regions； each said reduced thickness region being folded on itself； the inner diameter therewith the tube is originally extruded is that desired for a maximum diameter configuration； the thus obtained extruded tube being then thermoformed so as to cause the reduced thickness region edges to be approached to one another thereby providing a minimum diameter tube； the thermoforming method so handling the reduced thickness region as to cause the tube material forming its thickness to be folded inward the tube, to allow the tube edges to move one another.
