Chadbourne is losing the head of its corporate finance group Dennis Friedman and two other partners, Barbara Becker and Scott Kislin.
Friedman is one of the city's well-known rainmakers, with a practice acting for international investment banks, public and private issuers, internet companies, special committees and major equity funds.
He joined Chadbourne five years ago after walking out of Morrison & Foerster (MoFo) with three partners and about 12 associates, including Becker.
In the 1980s he was a partner at Shearman & Sterling, which is where he built up contacts with the investment bankers. He left in 1986 to work at brokerage firm Kidder Peabody and then at securities firm Painewebber, before joining MoFo in 1992.
Becker's practice focuses on securities offerings and M&A for technology and internet companies. Kislin also does M&A, as well as representing private equity funds, investment banks and venture capitalists.
The three partners were not available for comment, and Wesley Howell, the chairman of Gibson Dunn, would only issue a statement which read: "We are very pleased to have lawyers of their calibre as colleagues, and they have been joined by a number of associates.
"They strengthen and expand our already highly respected international corporate finance and M&A practices."
The New York office now has around 150 lawyers, with a corporate practice of around 250 spanning its London, Paris and 10 US offices.
In London this April, Gibson Dunn hired the former chief general counsel of investment bank Nomura to take the helm of its English law capability.
A legal headhunter in New York says the team is a great addition for Gibson Dunn.
He says: "[Friedman] has brought in quite a lot of business at Chadbourne, and he is a fairly significant M&A lawyer. He will probably add a lot of investment bank clients and work to Gibson Dunn."
Chadbourne's corporate group now numbers around 25 lawyers.