Philadelphia firm Cozen and O'Connor is opening offices in Chicago and San Francisco and bringing on board more than 30 additional lawyers.
The 420-lawyer firm is merging with Chicago-based firm Blatt Hammesfahr & Eaton to establish a practice there. The merger will add to Cozen's insurance practice, which is already one of the largest in the US, with 250 lawyers.
Cozen's clients include major US insurers such as AIG, Chubb Insurance and St Pauls. In London, the firm acts for Lloyd's syndicates including Hiscox and QBE.
Thirty lawyers will join from Blatt Hammesfahr, which also has offices in New York and Los Angeles which will combine with Cozen's existing presences.
London managing partner David Strawbridge says: "I think it's fairly well recognised that, save for New York City, Chicago is probably the second most important market for insurance generally.
"This is part of a strategy we have been unfolding which will put us in a position to cover the whole of the US with London market lawyers, by which I mean American lawyers with London clients. The only place where we had a bit of a gap was in the Chicago area - until now."
Although none of its lawyers are stationed in London, Blatt Hammesfahr has a tenancy at Lloyd's, and Cozen will take that over.
The managing partner at Chicago will be Robert Hammesfahr.
In San Francisco the firm is taking on three partners from local rival Booth & Banning. Norman Ronneberg and Cynthia Mitchell will head the office, which will also boast two associates from the San Diego office as well as Forrest Booth, a Booth & Banning partner who is moving his London-based practice to the new office.
Strawbridge says: "San Francisco was probably a case of taking advantage of an opportunity to move some quite experienced lawyers with the requisite know-how in the types of areas that fit in with our firm."
In London, Cozen has two US-qualified insurance lawyers and one English-qualified, and has an affiliation with City firm Radcliffes. It now has 15 offices in the US, and is planning to build on its transatlantic insurance capability.
Strawbridge says: "What we're finding with the consolidation in the insurance markets is that there are a lot of claims that come from London which also have American interests, and vice versa."