Abstract: In the United States, antitrust authorities rely heavily on numerical measures of local banking market concentration such as the Herfindahl Hirschmann Index to assess the likely competitive effects of proposed bank mergers and acquisitions. This approach to antitrust enforcement relies on two important assumptions: (1) that markets for at least some types of banking products are local in scope, and (2) that market concentration measures can serve as effective proxies for banks' abilities to extract monopoly rents. This paper uses balance sheet data from most banks operating in the United States in 1988, 1992, 1996, and 1999 to test these assumptions.

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