This paper considers a securities market in which orders are channeled through professional broker-dealers such as London's market makers or the large banks operating on continental exchanges. If these dual-capacity dealers can judge the motives behind their customers' orders, they can trade profitably on their own account (even if they cannot “front run,” that is, trade on their own account before executing a customer order). It is shown that the dealers have an incentive to satisfy roughly half of their customers' orders from their own inventory if they are sure that orders are liquidity-motivated and not based on inside information. As a result of dual-capacity dealing, transaction costs for liquidity-motivated traders in the aggregate fall, but they rise for those traders who are unable to convince any dealer that they have no inside information. The liquidity of the main market worsens, even though its effective liquidity for customers whose orders are partly filled from broker-dealer inventories improves.