Abstract: Polyurethane products having long-term biostability in combination with a low modulus of elasticity and a high ultimate tensile strength and other desireable mechanical properties which are particularly useful as leads for implantable pacemakers, as vascular grafts, as mammary prostheses, and as other products which are intended to be placed within the body of a mammal for extended periods with no substantial degradation of the product. The polyurethanes are prepared from the reaction of an organic diisocyanate, preferably an aliphatic or cycloaliphatic diisocyanate with polycarbonate glycol chained extended with diol, diamine or a mixture of diamine and alkanolamine.