The present invention provides embodiments of devices for causing weight loss in obese patients that are used to create a gastrointestinal bypass between a first region of the gastrointestinal tract and a second region of the gastrointestinal tract. The devices comprise an adjustable opening to adjust the fraction of food material passing through the gastrointestinal bypass. Also disclosed are methods for causing weight loss in obese patients comprising the step of creating an adjustable gastrointestinal bypass between a first region of the gastrointestinal tract and a second region of the gastrointestinal tract.
