<p>This work draws on the concept of hollowing and fortifying to examine the ability of an industry to self-regulate and how the stringency of a private regulatory institution works over time. Although previous research has shown that such institutions sustain themselves by balancing the reduction of stringency (gaining more support from industry participants) and increasing stringency (gaining more support from non-industry stakeholders), we present a historical case in which such institutions can also sustain by only increasing stringency. We conduct a longitudinal, qualitative analysis of news articles, official government records and other documents related to the Dutch Securities Institute, the self-regulatory organization in the Dutch securities industry. Our findings show how a self-regulatory institution can function through mere fortifying actions. The results also suggest a difference between stakeholders in their effectiveness to affect self-regulatory institutions.</p>