InsideCounsel, Jan. 2008 at 50, provides some data from the most recent law department survey of Altman Weil. The survey reports that “51.8% of outside fees go to the company’s top two firms” (See my post of March 24, 2005 on concentration of spending; and Dec. 17, 2007 on concentration efforts by law firms.).
This is not to say that the top-two law firms of a law department stay the same from year to year. A single large case may command huge sums for a firm during a year, but once that case ends, the firm slips back into the pack. Nor is it to say that the top-two firms ought to stay the same (See my post of Dec.16, 2005 on complacency as it afflicts incumbent firms; and Dec. 6, 2006 on retrograde effects of institutional knowledge.). All that said, most law departments have a small number of law firms that they regard as go-to firms for major undertakings.