Abstract: A flexible thermal jacket of a generally thin flat arcuate configuration, having a heat transfer face and an insulating face, for forming around and thermally contacting an organ and circulating a thermal fluid for controlling the temperature of the organ comprises, a first sheet of thin pliable flexible impervious material having an arcuate configuration defined by radially extending side edges and by inner and outer arcuate edges extending between the side edges, a second sheet substantially identical to the first sheet, the first sheet overlying and sealingly bonded to the second sheet along the arcuate edges thereof and along lines defining an elongated serpentine fluid passage extending throughout the area of the sheets between adjacent inlet and outlet ports opening outward from the outer arcuate edge adjacent one of the side edges of the sheet.