Simplifying procedures and improving legislation generally lead to a reduction in the compliance costs. The introduction of pre-filled tax returns clearly simplifies the tax compliance procedure. Before the introduction of pre-filled tax returns for personal income taxpayers in Slovenia, tax legislation was also modified. This paper presents the results of research into the compliance costs for personal income taxpayers before and after the simplification of the compliance procedure in Slovenia, irrespective of tax legislation itself not being simplified. The results indicate that pre-filled tax returns reduce compliance costs for personal income taxpayers by around 73%. Nevertheless, this is only a tentative estimate, since several assumptions are taken into account.