The invention relates to an implantable stomach prosthesis for surgically replacing or augmenting all or part of the antrum and/or pylorus of a stomach. The prosthesis controls the passage of food from the stomach to the small intestine. The prosthesis may be configured to churn ingested material and release it from the stomach through a prosthetic pyloric valve. At least one expandable member is arranged to be expanded to control the passage of food and/or to mimic the churning action of a patient's stomach. The prosthesis includes an outer support structure, a flexible inner member forming a conduit for the movement of material, and at least one expandable member located between the outer support structure and inner member. An implantable pump system is provided for inflating and deflating the expandable member(s).