In linguistics, the Gunning-Fog Index is a test designed to measure the readability of a sample of English text. The readability index is based on two criteria: a syntactic (the length of text sentences) and a semantic one (the number of polysyllabic words). The mathematical algorithm lets then establish the text difficulty to its potential reader. The resulting number is an indication of the number of years of formal education that a person requires in order to easily understand the text on the first reading. That is, if a passage has a fog index of 12, it has the reading level of a U.S. high school senior. The paper presents an attempt to adopt the index elaborated for English for measuring readability of texts written in Polish and shows the possibility of its implementation in the area of teaching Polish as a foreign language.