Abstract: Information on the ethnicity of the inhabitants in Czech Lands and subsequently also in the Czechoslovak Republic had an effect on political events at that time and other legal regulations were connected with them. Starting in 1880 the census began to determine the colloquial speech of those surveyed, from which the ethnic composition of the population of Predlitavsko was determined. Prior to 1880 the ethnicity of individuals in Austria-Hungary was not surveyed. The vagueness of the definition of colloquial speech contributed to strengthening the proportion of German-speakers to the detriment of others, which in the Czech lands meant mainly the Czech-Moravian-Slovak colloquial speech. During these First Czechoslovak Republic censuses (1921 and 1930) ethnicity was determined, mainly on the basis of maternal language. This method of determining ethnicity led to a number of conflicts, one of which went as far as the Highest Administrative Court. The rulings that the court issued in connection with determining the ethnicity of the population in a census are mentioned in the article. The court contributed to unifying the method used to determine ethnicity at this time.