Gender quotas aim to increase women's parliamentary representation. However, the effectiveness of quotas varies. This article explores this issue further by examining the case of Poland, where gender quotas were introduced in 2011. The Polish case presents an interesting puzzle. Although the overall number of women candidates increased almost twofold in comparison with the pre-quota period, this translated into only a slight increase in the number of women deputies in 2011 and 2015. Hence, the impact of quotas was limited. However, the partisan analysis shows that there was a significant variation among individual parties: whereas some parties promoted wholeheartedly women's access to political office, other parties did not facilitate it.By drawing on rational choice institutionalism, this article shows that institutions and preferences of political parties matter for the effectiveness of gender quotas. In the case of ineffective gender quota policy, political parties have a final say in women's parliamentary representation. 4 The literature points to a third institutional provision too, that is, the gender quota size. But it will not be examined here because it has remained the same in Poland (quota size of 35%). 5 Three parties, the Freedom Union, the Labour Union and the Democratic Left Alliance, introduced party quotas of at least 30% of each gender on candidates' lists for the 2001 election. 6 While there has been a steady increase in women's representation in the Sejm, the representation of women in the Senate has been very poor and remained at a low level of around 12% because of the majoritarian electoral system and the lack of gender quotas (see Gwiazda, 2016). 7 The elections of 1989 were semi-free for the Sejm where 65% of seats were reserved for the communists, but they were freely contested for a re-established Senate.