The experimental study of the vitality of the vocabulary of the Polish language is based on the example of the collection of names and definitions of men and women, which are used in the works of the outstanding Polish writer – Stefan Żeromski (1864 –1925). These words (a total of 864 words) were set against their vitality in the idiolect of the author of the article (born in the year 1932). Thereafter they were compared with the condition of the national vocabulary documented in two lexicons of the Polish language edited by W. Doroszewski (based on the canon of texts of the 19th century and the first half of the 20th century) and edited by S. Dubisz (incorporating the modern language). The experiment was based on the verification of the continuity of the life of words in the idiolect of the direct witness and participant in the long-term period of the life of the vocabulary. The conclusions, which were drawn up on the basis of this study, are as follows: the permanent vitality of words (lexical and semantic) is maintained during a longer period in the live public communication, as compared to the lexicons. 56.7% of the collection of words used in Żeromski’s letters were preserved in the author’s idiolect. Besides, during a period of more than 150 years, several caesuras (turning points) in the life of specified groups of the analyzed vocabulary were observed. The most expressive caesura occurred at the end of the 20th century and the beginning of the 21th century; during this period, the life of many words – including the live language of communication – which had been used before, discontinued. The external reasons are mainly the systemic changes, which took place after the World War II, and most recently – the civilization and cultural changes.