Disclosed are novel polymers derivatized with at least one --NO.sub.x group per 1200 atomic mass unit of the polymer. X is one or two. In one embodiment, the polymer is an S-nitrosylated polymer and is prepared by reacting a polythiolated polymer with a nitrosylating agent under conditions suitable for nitrosylating free thiol groups. The polymers of the present invention can be used to coat medical devices to deliver nitric oxide in vivo to treatment sites.