A knee immobilizer (10) including an elongate frame (12) having opposed convex and concave major surfaces for being positioned along a longitudinal axis of a dorsal aspect of a leg from above to below the knee, and a plurality of outwardly-extending retainers (12A-12D, 12E-12G) cooperating with the frame (12) for retaining the frame (12) on the leg. A lining (16) is provided forbeing positioned between the frame (12) and the leg when in use to cushion and protect the leg against direct contact with the frame (12). A spine (18) is provided for being positioned on the frame (12) substantially along an elongate axis of the frame (12) from above to below the knee and formed in an angle for maintaining the frame (12) in a predetermined rigid medical treatment extension position on the leg.