In recent years, both the SEC (2003) and the FASB (2004) [Securities and Exchange Commission, 2003. Study Pursuant to Section 108(d) of the Sarbanes-Oxley Act of 2002 on the Adoption by the United States Financial Reporting System of a Principles-Based Accounting System. Securities and Exchange Commission, Washington, DC; the Financial Accounting Standards Board, 2004. On the road to an objectives-oriented accounting system. Financial Accounting Series: The FASB Report (August 31), 1–5.] have indicated a need for accounting standards where principles are balanced by implementation guidance (i.e., a framework for exercising professional judgment). In this study, we take advantage of a jurisdictional split during 1996–2001 whereby the same economic event (i.e., an impairment in oil and gas assets) in the extractive petroleum industry was accounted for by “full cost” firms under a SEC standard (Regulation SX 4-10) which provides extensive implementation guidance, and by “successful efforts” firms under a FASB standard (SFAS No. 121) that provided relatively little guidance for implementing the standard. We utilize this jurisdictional split (which provides a natural quasi-experiment) to evaluate whether the differential guidance for recognizing impairment losses provided by Reg. SX (for “full cost” firms) and SFAS No. 121 (for “successful efforts” firms) is associated with differences in opportunistic reporting (as proxied by an association between the reported write-downs and managerial incentives to report opportunistically) and differences in the value relevance of the write-downs (as proxied by their association with stock returns). Our findings are inconclusive as to whether implementation guidance limits opportunistic reporting, but are broadly consistent with the notion that such guidance potentially undermines the value relevance of accounting information. Collectively, our findings provide accounting policy makers with an additional piece of evidence with which to evaluate the role of implementation guidance in standard setting.