The present invention provides an undercast liner for being applied to an anatomical shape of a patient and overlaid with a cast material. The undercast liner includes an elongate fabric having two opposing faces and an intermediate spacer area that both separates and interconnects the opposed faces. The liner is constructed at least in part of hydrophobic, water resistant monofilament yarn for providing enhanced water resistance, light weight, breathability and resistance to collapse and degradation due to moisture and bacteria during extended use of the liner. The liner has sufficient stretch in both a length-wise and width-wise direction to facilitate conforming the undercast liner around the anatomical shape without wrinkling during application.