In Croatia, Kosak families are Croats (from Ivanic-Grad area ). In the past century, relatively most of Croatian residents bearing this family name were born in Zagreb and in Ivanic-Grad area.
Prevalence
About 50 people with faimily name Kosak live in Croatia today, in 20 households. There were 80 of them in the middle of the past century, and their number decreased by 40 percent. They are only located in in 6 Croatian cities and 7 other places, mostly in Vezisce in Ivanic-Grad area (10), Zagreb (<10), Dugo Selo (<10), Grabric in Vrbovec area (<10), and in Petrinja (<10).
In the sources analysed in the project Acta Croatica so far, the name is mentioned over thirty times, the first time 1902 in source Hartmann's General List of Apartments in Zagreb.