In this paper we report on a composite index of international patent systems strength for 48 developing and industrialized countries annually from 1998 to 2011. Building upon earlier indices we develop a conceptual framework informed by transaction cost theory and derive measures which emphasize the importance of enforcement-related aspects of the patent system of countries. Findings reveal harmonization of the regulative aspects of patent protection internationally in the post-TRIPs era but not of overall national patent systems. The index should inform studies on the relationship between national patent systems and a range of international business and other phenomena.