Lower cost structures helped propel two firms onto The Am Law 100 for the first time: Lewis Brisbois Bisgaard & Smith, which has grown from a six-lawyer insurance defense shop in Los Angeles in 1979 to an 804-lawyer national practice, and Indianapolis’s Barnes & Thornburg, a 469-lawyer firm that has been snapping up big-firm litigators (and corporate partners) who don’t want big-firm billing rates.

“We were growing at a time when other law firms weren’t,” says Barnes & Thornburg managing partner Alan Levin, whose firm has launched five offices since 2009, including one in Los Angeles in 2011.

Also new to the list this year is Kilpatrick Townsend & Stockton, a product of the merger between Am Law 200 firms Kilpatrick Stockton of Atlanta and Townsend and Townsend and Crew of San Francisco [see "Combo Craze,"]. And Wilson Elser Moskowitz Edelman & Dicker returns to the list from a stint in the Second Hundred.

The four that dropped off The Am Law 100 include two high-end firms where growth has stalled: Boston’s Mintz, Levin, Cohn, Ferris, Glovsky and Popeo and Houston’s Bracewell & Giuliani. McKenna Long & Aldridge, which first moved onto The Am Law 100 a year ago at number 100, dropped off as well but will likely return in 2013 on the strength of its March merger with San Diego’s Luce, Forward, Hamilton & Scripps. The fourth firm to drop off is Husch Blackwell, which had appeared on The Am Law 100 each year after its formation in 2008 via the merger between Husch & Eppenberger and Blackwell Sanders.

Barnes & Thornburg’s elevation to The Am Law 100 follows four years of rapid expansion that pushed the firm from number 129 on the Second Hundred in 2008 to number ninety-nine on this year’s list. Revenues have grown more than 50 percent over that four-year period, from $197.5 million to $297.5 million. The growth hasn’t all been head count–driven; the firm’s revenue per lawyer jumped from $475,000 to $635,000 over those four years.

With a lower rate structure than most big competitors on the coasts, Barnes & Thornburg believed that the recession offered it a strategic opportunity. Building on core strengths in IP litigation, corporate law, and labor and employment, the firm opened offices in Atlanta, Minneapolis, Columbus, and Wilmington in 2009, then in Los Angeles last year with eight partners from Akin Gump Strauss Hauer & Feld. “A lot of the growth we’ve experienced is adding talent to those offices,” Levin says. The firm’s more established Chicago office has added partners from Greenberg Traurig and Dewey & LeBoeuf in the last 12 months.

Expansion has played a big role in Lewis Brisbois’s climb too. Initially an insurance defense shop, by the turn of the century Lewis Brisbois expanded into such areas as public finance and IP litigation. In 2002 it opened its first office outside California, in New York. In the past ten years, its head count has increased from 356 lawyers in 2002 to 804, spread across 21 offices in 11 states. “We clearly have grown a lot laterally,” firm chairman Robert Lewis says. Big areas of growth recently have included health care—medical malpractice, elder abuse, and ERISA—and labor and ­employment.

Wilson, Elser fits a profile similar to Lewis Brisbois in both size and client mix. Wilson, Elser is no stranger to The Am Law 100; it peaked at number 60 on the list in 1994. The firm fell off after 2007 but is back this year as number 94.