This study analyzes the relationship between the specialization of a region in certain industries and the efficiency of the region in generating new knowledge. The efficiency measure is constructed by relating regional R&D input and output. An inversely u-shaped relationship is found between regional specialization and R&D efficiency, indicating the presence of externalities of both Marshall and Jacobs' type. Further factors influencing efficiency are spillovers within the private sector as well as from public research institutions. The impact of both the specialization and the additional factors is, however, different for regions at different efficiency levels.