We examined the discursive processes through which a new professional role identity for registered nurses was legitimized by analysing introductory textbooks over time. We theorize five ways of rhetorically legitimizing a new professional role identity: naturalizing the past, normalizing new meanings, altering identity referents, connecting with the institutional environment, and referencing authority. In contrast to previous research focused on legitimizing new practices, we contribute to the institutional literature by showing that legitimizing a professional role identity requires the incremental development of new arguments where the past is not delegitimized. Our findings also indicate that instead of a progression from moral and pragmatic legitimacy to cognitive legitimacy, legitimizing a new role identity may focus only on moral legitimacy. Finally, our study highlights the importance of interactions between the professional task environment and the wider institutional environment as part of the process of legitimizing a professional role identity. Copyright (c) 2009 Blackwell Publishing Ltd and Society for the Advancement of Management Studies.