This study is dedicated to a really serious problem, because the question of ethnicity at the census in the Teschen Silesia in the period between World Wars was frequently topic of the polemic between Poland and Czechoslovakia. After the collapse of Austria-Hungary Empire the Teschen Silesia was divided between Poland and Czechoslovakia. On the Czechoslovak teritorry stayed a Polish minority, but there is dispute, how large. One of the specifics of the region was a certain number of people, who wasn't sure about their ethnicity. These people, called Slonzaks, spoke the Polish dialect, but they lacked the Polish national awareness and they were at the Austrian Census added to the Poles. The Czech administration supported of course this group of people for the purpose to turn them into Czechs and therefore at the census there was placed a special slonzak's category, which registered with a large number of people, who spoke the Polish dialect. Conscious Poles understood it as an act of national oppression, and quite right, because in fact this category hadn't any other sense, than to reduce the number of the Poles at the census.