We evaluate the impact of the Sarbanes-Oxley Act (SOX) on shareholders by studying the lobbying behavior of investors and corporate insiders in order to affect the final implemented rules under SOX. Investors lobbied overwhelmingly in favor of strict implementation of SOX, while corporate insiders and business groups lobbied against strict implementation. We identify firms most affected by the law as those whose insiders lobbied against strict implementation. Such firms appear to be characterized by agency problems, rather than motivated by concerns over compliance costs. Cumulative stock returns during the five and a half months leading up to SOX passage were approximately 7% higher for corporations whose insiders lobbied against SOX disclosure-related provisions than for similar non-lobbying firms, consistent with an expectation that SOX would reduce agency problems. Analysis of returns in the post-passage implementation period suggests that investors' positive expectations with regards to the effects of these provisions were warranted.