ENThe final results of parliamentary elections may rapidly become out of date, especially in some parliaments in the East Central European countries. During the term parliamentary mandates can expire, causing MPs’ replacement, or some deputies may change their party affiliation. It results in a situation in which the voters’ will is not respected for the whole duration of the parliamentary term. The main contribution of this chapter is to complement the current state of research on party systems with an analysis of its dynamic between the parliamentary elections through conceptualisation and measurement of the phenomenon of intraparliamentary volatility and its influence on the intraparliamentary gender balance. The objective of this chapter is to understand the causes and consequences of this intraparliamentary volatility and to investigate its size, focusing on the gender factor. The main research question is about the differences and similarities between the female and male MPs’ decisions on party-switching or replacements, as well as its consequences for the party system. Three cases and nine terms were selected for the study: Czech Republic (Poslanecká sněmovna, terms of office: 2010–2013, 2013–2017, 2017–2021), Lithuania (Seimas, terms of office: 2008–2012, 2012–2016, 2016–2020), and Poland (Sejm, terms of office: 2007–2011, 2011–2015, 2015–2019). [p. 2].