A method and apparatus for reducing the size of the stomach cavity in humans. A collapsed balloon is packaged in water soluble material and releasably attached to a filler tube. The balloon is inserted into the cavity through the mouth and esophagus and pumped up to a desired volume through the tube after water soluble material packaging has dissolved and the balloon deployed. After stomach fluids also dissolve the releasing mechanism, the filler tube is removed from the stomach. The balloon remains in the stomach for the required dieting time. Its volume prevents the usual ingestion of food into the stomach by obese patients, causing them to feel full, and reducing the desire for food. At the end of this period a puncturing mechanism is introduced into the stomach, punctures the balloon, grasps it and the balloon is removed through the esophagus and mouth.