Not content with his rousing turnaround of Mellon BankCorp., Frank V. Cahouet has engineered a series of audacious acquisitions that will transform the Pittsburgh-based company into a fee-driven leviathan.

The ball got rolling in September, when Mr. Cahouet plunked down $1.45 billion for Boston Co., with $9.2 billion of banking assets and nearly $300 billion of trust assets. Then, in a stunning move, Mr. Cahouet in December agreed to pay $1.85 billion for Dreyfus Corp., the nation's sixth largest mutual fund company.

Pro-forma numbers show the dimensions of these efforts: Including Boston Co. and Dreyfus, Mellon ranked 13th among U.S. banking companies in revenues Q despite ranking 21st in banking assets.

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