Pillsbury Winthrop LLP and Shaw Pittman LLP have agreed to a merger that would create one of the 20 largest law firms in the country as ranked by annual revenues.

The new firm, to be called Pillsbury Winthrop Shaw Pittman LLP, will have 900 attorneys in 16 offices, including two in Los Angeles.

Based in San Francisco, Pillsbury Winthrop, focused on corporate finance, has more than 100 attorneys in its Los Angeles offices. The firm, which ranked No. 30 on American Lawyer magazine’s AmLaw 100, generated revenues of $423.5 million in 2003.

Shaw Pittman, a Washington, D.C.-based technology and government firm ranked No. 99 last year on the AmLaw list, had 2003 revenues of $189.5 million. The firm, which has an office in Northern California, closed its Los Angeles office in the past few years.

Mary Cranston, Pillsbury Winthrop’s chairwoman, splits her time between San Francisco and Los Angeles and will head the new firm.

In Los Angeles, the merger would bring needed expertise in real estate investment trusts, securities, government relations and outsourcing, said Mike Finnegan, a Los Angeles partner who sits on Pillsbury’s firm-wide managing board.

“Outsourcing has been a growing business phenomenon and legal area,” he said. “Shaw Pittman has the single nationally recognized practice group in that area.”

He could not say whether the merger would result in growth or layoffs at the firm’s local offices. “Exactly how it ferrets out is yet to be determined,” he said.

The merger is expected to close by mid-Spring, pending approval by the partners.