A hypothermia treatment sack comprising a top sheet and a bottom sheet releasably connecting along their perimeters to form a patient receiving cavity between the top and bottom sheets while allowing for patient access from any location around the perimeter. The top and bottom sheets include a weatherproof exterior side for repelling wind and water, and a heat reflective interior side for preventing heat from escaping. A plurality of self-activating heating pads are carried in the top sheet for producing heat when exposed to air. The heating pads have a first side exposed on the weatherproof exterior side of the top sheet to provide for a generally unrestricted airflow to the heating pads, and a second side exposed on the heat reflective interior side of the top sheet for radiating heat directly into the patient receiving cavity.