In this article, I argue that Westminster parliamentary systems encourage large opposition parties to replace their leaders between elections. Parliamentary system structures how parties compete over legislative outcomes. In Westminster systems, the government’s dominance in the legislative process promotes an adversarial government–opposition relationship. Subsequently, large opposition parties’ electoral prospects are tied to their ability to discredit the government’s policy agenda. Since this responsibility falls to party leaders, leaders of large opposition parties directly affect their parties’ electoral prospects, and parties are more motivated to replace those who are ineffective in damaging the government’s credibility. Therefore, leaders of large opposition parties in Westminster systems carry a higher risk of replacement than their counterparts in other parliamentary systems. I construct an original data set on party leadership turnover in 14 established parliamentary democracies. Results from Cox proportional hazard models support my claim and suggest that institution influences intraparty dynamics.