Abstract: An inflatable cast system for immobilizing a lower extremity of a human. The system includes a pair of outer shell members shaped to conform to the lateral and medial sides of the lower extremity, respectively. A base member is fastened to the shell members to form a substantially U-shaped stirrup member. The upstanding sidewalls of the shell members are maintained in position relative to the lower extremity by a plurality of elongated, circumferentially extending, longitudinally spaced, flexible fastening straps. A pair of inflatable air bags are attached to inside surfaces of the shell members. The air bags include at least one chamber and a valve that is sealed along one edge of the air bag and is adapted to allow the chamber to be automatically sealed once the brace is attached to the extremity of the user. A unidirectional air pump is provided to allow the chamber of the air bag to be selectively inflated and deflated.