Abstract: We use the Business Roundtable’s challenge to the Securities and Exchange Commission’s (SEC’s) 2010 proxy access rule as a natural experiment to measure the value of shareholder proxy access. We find that firms that would have been most vulnerable to proxy access, as measured by institutional ownership and activist institutional ownership, lost value on October 4, 2010, when the SEC unexpectedly announced that it would delay implementation of the rule in response to the Business Roundtable’s challenge. We examine intraday returns and find that the loss of value occurred just after the SEC’s announcement on October 4. We find similar results for July 22, 2011, when the U.S. Court of Appeals for the District of Columbia Circuit ruled in favor of the Business Roundtable. These findings are consistent with the view that financial markets placed a positive value on shareholder access, as implemented in the SEC’s 2010 rule.