An anti-obesity balloon and a placement system for the balloon includes a balloon (10) with a needle-pierceable, self-sealing plug (20) and an insertion catheter (12) having a needle (26) at its distal end. The needle (26) is movable between an extended position in which it protrudes distally of the insertion catheter (12) and a retracted position in which it is withdrawn interiorly of the insertion catheter (12). A handle arrangement (14) is provided at the proximal end of the insertion catheter to control the position of the needle (26). Means are provided for aspirating (16) and inflating (68) the balloon. The balloon structure (10) and insertion catheter structure (12) protect and enclose the sharp tip of the needle at all times during insertion and withdrawal of the insertion catheter (12). The insertion catheter (12) is detachable from the balloon (10), after the balloon (10) has been inflated in the patient's stomach, in a manner which imposes no force or load on the balloon.