In Croatia, Trnjak families are predominantly Croats (from Cazins Region (Bosnia) ), rarely Serbs. In the past century, relatively most of Croatian residents bearing this family name were born in Vukovar area and in Cazins Region, Bosnia (Sanski Most area).
Prevalence
Only a few people with faimily name Trnjak live in Croatia today, in four households only. They are located in only threee naselja i to in Vukovar (<5), Ludbreg (<5), and in Borovo Naselje (<5).
In the sources analysed in the project Acta Croatica so far, the name is mentioned over ten times, the first time 1917 in source List of Dignitaries, Officials and Public Servants of the Kingdom of Croatia-Slavonia.