Purpose In this paper we analyse the effect on unconditional conservatism of the mandatory adoption of International Financial Reporting Standards (IFRS) by the European listed firms in January 2005. Under the hypothesis that accounting regulation influences the accounting conservatism, we use a non-market-based measure of unconditional conservatism – the accrual-based measure proposed by Givoly and Hayn (2000) – to test this effect, controlling for the other determinants of the unconditional conservatism found in the accounting literature. Design/methodology/approach We use a panel data of 10 years and 96 non-financial listed firms in the Spanish stock market in which the differences between local GAAP and IFRS are more important. A pre-estimation analysis of the data reveals that GLS with random effects is the correct estimation procedure. However, to try to deal with the likely endogeneity in the set of variables, the authors perform an estimate with a dynamic estimator for panels with few periods and many individuals where the independent variables are not strictly exogenous. Findings As expected, results show evidence that support a significant reduction on the unconditional conservatism of firms in the sample due to the adoption of IFRS. This evidence is relevant to equity market, debt market and corporate governance users of the financial information, and also for the policymakers who can assess the effects of their mandate. Research limitations/implications Results shown in this paper have all the limitations of system-, country-, sample- and event-specific studies but, along with many others drawn in alternative contexts, may help to correctly understand both the time-evolution and cross-sectional country differences of firms’ unconditional conservatism. Originality/value The study represents the first analysis of the effect of the adoption of IFRS on unconditional conservatism of the European listed companies using a non-market accrual-based measure. Results are not influenced by the dynamics of the stock market and, by comparison, allow us to analyse this influence in results provided by using market-based measures of the unconditional accounting conservatism provided by previous literature.