This article analyses how a firm's returns are affected when a bank becomes a large blockholder. We investigate this issue by taking into consideration the types of blockholders that build coalitions with banks in order to control a firm. We find that the effect on a firm's returns is negative when a bank buys the largest stake and forms coalitions with other banks. However, this negative effect does not apply in other situations. We underscore our theoretical conjectures based on an empirical analysis of a panel dataset comprising a representative sample of listed and unlisted Spanish firms over the period 1996 to 2000.