Lichen diversity in the City of Košice was studied investigating lichen diversity on 160 trees distributed over 32 sites. Its relationship to pollution sources (steel plant, thermal power station) were determined by calculating the IAP (Index of Atmospheric Purity). A total of 60 lichen species was found, 30 of which are endangered. The rare species Parmelina quercina is reported for the first time. The lichen diversity in the City of Košice is decreasing as we move closer to the sources of pollution and also in the southern part of the city. An intermediate level of diversity was identified in residential areas, parks and forest parks. The highest diversity was found in the recreational area at the forest edge, Bankov. Keywords: lichen distribution, endangered species, eastern Slovakia, air pollution, Central Europe.