Kos families are almost entirely Croats and they are mostly from Novi Marof area. According to some sources they are from middle Istria, very rarely Serbs. In the past century, relatively most of Croatian residents bearing this family name were born in Zagreb and Varazdin. In Nespeš in Sveti Ivan Zelina every third inhabitant had the family name Kos.
About 4600 people with faimily name Kos live in Croatia today, in 1900 households. There were 4700 of them in the middle of the past century, and their number remains constant. They are located in all Croatian counties, in 87 cities and 518 other places, mostly in Zagreb (895), Varazdin (135), Velika Gorica (115), Rijeka (90), and in Labin (75).