We provide a first attempt to include off-balance sheet, implicit insurance to SIFIs into a consistent assessment of fiscal sustainability, for 27 countries of the European Union. We first calculate tax gaps a ̀ la Blanchard (1990) and Blanchard et al. (1990). We then introduce two alternative measures of implicit off-balance sheet liabilities related to the risk of a systemic bank crisis. The first one relies on microeconomic data at the bank level. The second one is based on econometric estimations of the probability and the cost of a systemic banking crisis. The former approach provides an upper evaluation of the fiscal cost of systemic banking crises, whereas the latter one provides a lower one. Hence we believe that the combined use of...