Chromosome numbers for 23 taxa of Hieracium L. from Bulgaria, Greece, Poland and Slovakia are given and their metaphase plates are illustrated. The ploidy level of 8 taxa was also confirmed by flow cytometry. Chromosome numbers are published for the first time for Hieracium bracteolatum subsp. koracis (Boiss.) Zahn (4x), H. marmoreum Pančić & Vis. (3x), H. ossaeum Zahn (3x), H. sartorianum Boiss. & Heldr. (3x), H. sericophyllum Nejčeff & Zahn (3x) as well as for five other undescribed species.