Over the last 30 years researchers have examined the link between performance and the degree of internationalization, reporting inconsistent and contradictory results. This paper, by performing Data Envelopment Analysis (DEA), tries to contribute to the existing literature by investigating whether firms' internationalization levels have an impact on their performance. Using a sample of ten transnational corporations from South-East Europe the paper provides information regarding their efficiency levels. Finally, using the "Transnationality Index" (TNI) provided by UNCTAD in order to capture the levels of internationalisation, our results reveal that there is a positive influence on firms' performance.