An undergarment for use with a cast tape to form a hip spica cast, such as a pantaloon-type hip spica cast. The cast includes at least one layer of a 3D spacer synthetic padding material formed into a fabric of monofilament yarns and fabricated into an anatomically correct undergarment structure for being positioned around a torso and residing directly against the skin of a hip spica cast patient. At least one fabric perineal panel is attached to the undergarment in the perineal area of the undergarment and adapted for being positioned between the patient and an overlying cast material defining a perineal opening in the cast.
