A prosthesis (13) for implanting in an upper stomach to prevent gastric reflux in an esophagus comprising a tube (17) made of a biocompatible polymer that is resistant to gastric acid, the tube having an upper end and a lower end, a length, and a generally constant diameter along the entire length thereof, the upper end having means (16) for securing to the upper stomach, the lower end having at least one slit (18) to facilitate inversion of the tube during vomiting but to facilitate collapsing of the tube under pressure generated by gastric reflux.