This paper empirically investigates the relationship between managerial entrenchment and agency costs for a large sample of UK firms over the period 1999-2005. To measure managerial entrenchment, we use detailed information on ownership and board structures and managerial compensation. We develop a managerial entrenchment index, which captures the extent to which managers have the ability and incentives to expropriate wealth from shareholders. Our findings, which are based on a dynamic panel data analysis, show that there is a strong negative relationship between managerial entrenchment and our inverse proxy for agency costs, namely asset turnover ratio. There is also evidence that short-term debt and dividend payments work as effective corporate governance devices for UK firms. Finally, our findings reveal that agency costs are persistent over time. The results are robust to a number of alternative specifications, including varying measures of managerial entrenchment and agency costs.