Abstract: A double lumen cannula for hemodialysis and similar applications such as haemoperfusion comprises elongated flexible tubular members, disposed one within the other, for semi-permanent insertion into the sub-clavian vein of a patient, leaving semi-permanent access thereto for repeated hemodialysis treatments. The cannula is formed of an elongated flexible outer member, having a smooth exterior surface, and made up of an elongated main portion, a convergent, constricting portion and a narrower distal end portion which is tapered at the tip to fit over a Seldinger guide wire. There is also an inner member having a diameter substantially the same as that of the distal end portion. The inner member is insertable into the outer member to come into sealable engagement with the interior walls of the constricting portion, so as to form a continuous inner lumen with the distal end portion of the outer member, and is removable therefrom for removal of clot or replacement purposes.