The aim of this paper is twofold. The first aim is to identify the factors behind the growth of medium-sized enterprises achieved by comparing them with small enterprises. Secondly, among the factors that determine the economic performance of the medium- size enterprise, this study also aims to identify the role of entrepreneurial orientation. A database of economic-financial information is developed and the results of a survey of medium-sized enterprises are analysed in order to investigate the two aims of this study. The results of the econometric analysis highlight some of the main factors determining growth of medium-sized enterprises. Significant among these are proactivity, risk taking posture, and investment aimed at growth.