The mechanical properties of poly(urethaneimides) (PUI) were compared with known biomedical polyurethanes. The resistance of PUI to extraction with hexane and water and to sterilization with ethylene oxide or water vapor was stated. Water sorption and the permeability of water vapor, oxygen, and carbon dioxide through PUI foils were investigated. In blood compatibility, the PUI foils were similar to cellulose material Cuprophane, used in dialysis. Preliminary in vivo testing of PUI implanted in rats indicated good biocompatibility of this polymer.