Apparatus for altering the body temperature of a patient comprises an enclosure defining an interior space for receiving at least a portion of a patient's body therein. The enclosure is constructed for transferring a heat transfer liquid into direct contact with the portion of the patient's body received in the enclosure to promote heat transfer between the patient's body and the heat transfer liquid. A drain for draining the heat transfer liquid from the interior space of the enclosure is at least partially disposed beneath the enclosure. A hold-open inhibits occlusion of the drain. A weir in fluid communication with the drain maintains the heat transfer liquid at a predetermined height in the enclosure. A control system is preprogrammed with a target temperature for the body temperature of the patient.