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Due to the limited distribution of the Corncrake (Crex crex) in the Czech Republic, annual monitoring of its abundance was established in selected areas already during the 1990s. From 2005 to 2011, more extensive monitoring was carried out in three-year intervals in Special Protection Areas and other important sites for this species. In 2020, monitoring of Corncrake abundance in the Czech Republic was performed in 40 regions including all previously known centres of its distribution. Counts were carried out in two time periods between the end of May and the beginning of July. The total abundance of the Corncrake in the Czech Republic, based on the results of monitoring and additional records from the databases NDOP, Birds.cz, and eBird, was estimated at 1,200–1,400 calling males. We quantified a 35% decrease of abundance compared to the previous period of large-scale monitoring in 2011. The mean altitude of localities with newly (within a season) recorded calling males increased during the season, probably as a consequence of delayed development of vegetation at higher elevations. Highest numbers of Corncrakes in regularly monitored areas were recorded in the period 2000–2003 after a strong increase during the 1990s. Since 2004, fluctuations in abundance were observed with gradual decreasing trend, while a marked decline was recorded in some regions after 2015. The possible reasons of this decline include expansion of pastures at the expense of meadows, and housing development. Long-term drought after 2015 resulting in delayed development of vegetation during spring was probably also one of the reasons of declining Corncrake abundance in some regions. [ABSTRACT FROM AUTHOR]