Dual valve, flexible sheath and method

A large-diameter sheath and assembly for use in introducing a catheter or other medical device into a vessel of the body of a patient. The sheath comprises an elongate sheath tube formed of a flexible material which has proximal and distal extremities and a passage extending therethrough of a predetermined maximum diameter. The sheath tube may either be expandable, with the distal extremity folded longitudinally to a smaller folded diameter, or of a constant diameter. If expandable, the sheath tube may be self-expanding or may be reinforced with a self-expanding wire or expandable stents. A backflow adapter is secured to the proximal extremity of the elongate sheath tube.