Abstract: A double lumened catheter defined by inner and outer cannulas having spaced apart walls defining a fluid passageway therebetween. The outer cannula is sharpened at its outer end for ease in puncturing a blood vessel. The inner cannula is extendible past the sharpened end of the outer cannula and is bent at an angle toward the inner wall of the outer cannula. Engagement of the bent portion of the inner cannula with the sharpened front end of the outer cannula's inner wall prevents damage to the blood vessel after insertion of the double lumened catheter thereinto. After insertion, blood may be dialyzed by with-drawing blood through the space between the cannulas and returned via the inner cannula.