We analyse the extent to which managers exercise discretion under fair value accounting and the value relevance of these disclosures. Utilising a sample of firms that apply the UK fair value pension accounting standard, (FRS-17), we examine the main determinants of the assumptions managers use to arrive at pension scheme valuation. Despite little variation in the underlying economic inputs, differences in stated assumptions across companies, auditors and actuaries are significant. Managers display considerable variation in conservatism when implementing fair value accounting and this variation is related to scheme-specific characteristics, such as asset allocation and pension solvency. Although the disclosed amounts are found to be value relevant, the observed inconsistency in reporting across firms brings into question the efficacy of fair value accounting for assessing corporate risk.