In this article I respond to the tendency of the law to approach 'the public interest' as a legal test, thereby drawing the criticism that this narrow notion of what purports to be in the "public" interest is wholly disconnected from the views of actual publics, and lacks social legitimacy. On the other hand, to simply extrapolate outputs from public engagement work into policy (or indeed law) is equally problematic, and risks being at best ineffective and at worst reinforcing existing inequalities. Given this apparent disconnect between these conceptions of the public interest, and the shortfalls inherent in each, this article scrutinises this disjuncture. I argue that the application of a "processual lens" to the construction of the legal and regulatory role of the public interest sheds light on how legal notions of the public interest, and attitudes of actual publics towards data sharing, might be reconciled. I characterise this processual approach as being iterative and flexible, specifically drawing attention to the way that multiple actors, processes and interests interact, change and evolve over time in the health research endeavour. This approach is elaborated through two case studies that illustrate how the public interest appears in law (broadly conceived). Its application provides novel insights into the ways in which the public interest can be crafted within and beyond the law to better inform the development of health research regulation.