A prosthesis for use as an anti-reflux device or anti-obesity device within a gastro-intestinal tract of a living organism, said tract comprising an entrance end, an exit end and a wall defining a passageway, of generally circular cross-section having a diameter, along which fluid compositions such as masticated food, digestive secretions and/or mixtures thereof are conveyed within the living organism in a direction towards the exit end of the tract; said prosthesis consisting of a hollow elongate body, having a proximal end and a distal end, wherein said hollow elongate body consists of: a) an elastic portion having a normal undistorted shape comprising a wall having an inner surface and an outer surface that together define a cylindrical tube, having an outer diameter d, wherein said wall is formed from an elastic spring embedded within a biocompatible plastics material, wherein said elastic portion extends from the proximal end for a distance of less than 50% of the length of the hollow elongate body, wherein said elastic portion is adapted such that when said prosthesis is positioned for use within the gastro-intestinal tract said outer surface of said elastic portion contacts said wall of said tract and said inner surface of said elastic portion defines a cylindrical passageway concentric to and within the tract through which fluid compositions flowing within the tract towards the exit end may enter from the proximal end and flow towards and exit the distal end, and b) a flexible portion, comprising a wall having an inner surface and an outer surface that together define a tube having a passageway and consisting of a biocompatible plastics material, joined integrally to and extending from the elastic portion and terminating at the distal end of the hollow elongate tube, wherein said flexible portion is adapted such that when said prosthesis is positioned for use within the gastro-intestinal tract fluid compositions flowing within the cylindrical passageway of the elastic portion towards the exit end of the tract enter the passageway of the flexible portion and flow towards and exit the distal end; wherein said elastic portion has an outer diameter d which, in its normal undistorted shape, is greater than said diameter of said passageway of said tract at the position in said tract where during use said outer surface of said wall of elastic portion of said prosthesis contacts said wall of said tract; characterised in that said elastic spring in said elastic portion consists of a single helical spring, having two ends and consisting of from 2 to 20 rings between said ends, wherein each of said rings is concentric with the cylindrical tube of the elastic portion; and optionally wherein the elastic portion further comprises a layer of biocompatible plastics netting material attached to the outer surface of the wall. a) an elastic portion having a normal undistorted shape comprising a wall having an inner surface and an outer surface that together define a cylindrical tube, having an outer diameter d, wherein said wall is formed from an elastic spring embedded within a biocompatible plastics material, wherein said elastic portion extends from the proximal end for a distance of less than 50% of the length of the hollow elongate body, wherein said elastic portion is adapted such that when said prosthesis is positioned for use within the gastro-intestinal tract said outer surface of said elastic portion contacts said wall of said tract and said inner surface of said elastic portion defines a cylindrical passageway concentric to and within the tract through which fluid compositions flowing within the tract towards the exit end may enter from the proximal end and flow towards and exit the distal end, and b) a flexible portion, comprising a wall having an inner surface and an outer surface that together define a tube having a passageway and consisting of a biocompatible plastics material, joined integrally to and extending from the elastic portion and terminating at the distal end of the hollow elongate tube, wherein said flexible portion is adapted such that when said prosthesis is positioned for use within the gastro-intestinal tract fluid compositions flowing within the cylindrical passageway of the elastic portion towards the exit end of the tract enter the passageway of the flexible portion and flow towards and exit the distal end;