(FORTUNE Magazine) – By all accounts it was the perfect Silicon Valley wedding. The venue was beautiful--a historic mansion in tony Woodside. Equally beautiful were the people. Matthew Sonsini, son of Larry Sonsini, chief rainmaker at the Valley's most powerful law firm and one of the tech industry's savviest dealmakers, was marrying Lisa Sobrato, the daughter of a prominent Valley real estate developer. "Just about anybody who was anybody in Silicon Valley came to this wedding," says LSI Logic CEO Wilfred Corrigan, who attended. Indeed, the guest list for the affair last year included Sun Microsystems CEO Scott McNealy, legendary venture capitalist Don Valentine of Sequoia Capital, and Ken Oshman, founder of Rolm--all clients of the groom's father.

Welcome to the world of Larry Sonsini, head of Wilson Sonsini Goodrich & Rosati, the biggest and most plugged-in high-tech law firm in the country. WSGR represents 45% of the 150 largest public companies in the Valley, making it the go-to law firm for anything resembling a tech deal. So highly do its clients value the firm, and trust it, that many don't even mind when it also represents bitter rivals. WSGR is about to branch out from its Palo Alto headquarters to other U.S. high-tech centers, starting with metropolitan Seattle. The move into Bill Gates' back yard gives piquancy to the power that the firm derives from its pivotal role in the anti-Microsoft movement. WSGR star litigator Gary Reback is credited not only with instigating the government's Microsoft investigations but also with giving the Department of Justice some of its best ammunition in its current lawsuit against the software giant. Bill Gates was not at the Sonsini wedding. Neither was Intel Chairman Andy Grove, Bill's Valley ally.

For all these reasons and more, having Larry Sonsini as your lawyer is the ultimate Silicon Valley status symbol. CEOs actually brag at cocktail parties about being represented by him. Last year WSGR took public 44 companies--more than any other firm in the country, according to Securities Data, a Newark, N.J., research company. In the past two months alone, the firm has been a key player in such headline-grabbing tech deals as Disney's partial acquisition of WSGR client Infoseek and BankBoston's merger with client Robertson Stephens, a leading investment bank in the technology sector. Sonsini himself has handled some of the most celebrated technology deals to date: He took Apple public in 1980 and Netscape in 1995, and negotiated the $4.8 billion merger of Cisco Systems with WSGR client Stratacom in 1996, one of the biggest mergers in Silicon Valley history. "Someone referred to Larry as the consigliere of Silicon Valley," says John Doerr, a partner at the leading venture capital firm Kleiner Perkins Caufield & Byers. "He doesn't sit on the board of most companies, but in many ways he's got a closer relationship [with the CEOs] than the people who do."

None of this is what you would expect when you meet 57-year-old Larry Sonsini. Of average build and wont to dress in dark Brioni suits, Sonsini is friendly but reserved, resembling less a glad-handing dealmaker than an empathetic psychoanalyst who listens intently because he really wants to understand what you're saying. "People trust Larry Sonsini. He has a very calming, urbane style," says Doerr, who met Sonsini in 1981 while helping Sun Microsystems get started. "I've never heard him raise his voice. In fact, he lowers his voice. People pay more attention."

Much of Sonsini's appeal as a lawyer comes from who's listening when he speaks. Clients who pay Sonsini's hourly rate of more than $400 are buying not just expert lawyering but a web of relationships. Mayfield Fund general partner Kevin Fong, for instance, looks at over 2,000 business plans a year, and a word from the right person can help him decide which to invest in. He says, "If Larry Sonsini gives me a call and says, 'Kevin, you really need to look at this plan,' you will get my attention."

Important as such relationships are, the key to success for both the firm and its clients has been its own unique business plan. Founded in 1961 by John Wilson--he hired Sonsini straight out of Berkeley's Boalt Hall law school in 1966--the firm catered exclusively to the growing number of high-tech companies in the Valley, adding services as the clients got bigger and their needs became more sophisticated. Wilson had strong ties to the venture community; he introduced the two men who in 1969 started the Mayfield Fund, then and now a leading source of venture capital. While he recognized the potential of the startups, he understood they needed capital both to build their businesses and to pay their legal bills. The solution: The firm would not only introduce entrepreneurs to its venture capitalist friends but also help make its clients' business plans more attractive to investors. Frank Quattrone, until this August the CEO of the investment bank Deutsche Bank Securities Tech Group in Menlo Park, Calif., says, "I've been amazed at how many hours some of those attorneys spend with startup companies on things that I would have thought were much more business oriented than legal." WSGR sometimes invests in its clients too, a practice that has proved quite lucrative. WSGR estimates it has made $25 million on such investments in the past decade. In all, revenues last year were $174 million, up from $146 million in 1996 and nearly triple the level of five years ago.

WSGR's conspicuous success has raised eyebrows, particularly over the ethics of its representation of companies that compete against one another. Clients do seem to put an unusual degree of trust in the firm. They routinely turn to WSGR lawyers for business-strategy advice, sharing highly sensitive information in the process, at the same time that the firm might be engaged in similar consultations with their direct competitors. Amazingly, clients are fine with this. To keep WSGR in a deal, some clients sign conflict-of-interest waivers--and stick with the firm even when Larry Sonsini chooses to represent the opposing side. Case in point: In 1995 the firm's client Seagate decided to acquire Conner Peripherals, another WSGR client. Although either company could have shut out WSGR altogether, the firm ended up representing Seagate, and CEO Finis Conner signed a waiver because, says Sonsini, "they felt it would go more efficiently." Conner, now CEO of Conner Technology, echoes the sentiment almost exactly: "Having him in the deal was okay with me because I felt he would add to it," he says. "He is very sensible about things. It all went extremely well."

Up north on the Microsoft campus, opinions of Sonsini would run strongly in the opposite direction. WSGR senior partner Gary Reback probably holds the title as Bill Gates' chief antagonist, the feds included. Reback believes Microsoft should be dismembered a la Standard Oil, and he spends 20% of his time trying to make this happen. He has bolstered the government's antitrust cases against the company with a blizzard of white papers, briefs, and letters written on behalf of his clients--some anonymous; others, like Netscape, decidedly not--all seeking to show how Microsoft thwarts competition.

Many trace the government's current antitrust case to a white paper Reback wrote in 1994 opposing Microsoft's proposed acquisition of Intuit, the personal-finance software company. Reback argued that the deal would kill competition in that market, and he must have been persuasive: The acquisition was blocked, and the government has been interested in Microsoft ever since. "This is about the investment climate in Silicon Valley," says Reback. "When Microsoft dominates a market, funding stops for entrepreneurs. The capitalist system doesn't work the way it's supposed to in that it doesn't provide us with the best ideas."

Given WSGR's contentious relationship with its northern neighbor, it's more than a little ironic that the firm plans to open its first branch office later this year in Kirkland, Wash., just a few miles from Microsoft's headquarters in Redmond. While this may look like a great way to keep tabs on the enemy or compete with Microsoft to find promising startups in the area, the move is motivated more by the fact that the partner who will head the new office is a homesick Washingtonian. Regardless of the reason, WSGR's presence is likely be a boon to Washington entrepreneurs. The Mayfield Fund's Fong, for one, thinks Valley investors will be more comfortable putting their money in Washington startups if a WSGR lawyer is involved in the deal. More offices are already being considered; the next is expected to be in Austin, Texas.

In light of the expansion, Sonsini's sphere of influence seems almost certain to keep growing. Indeed, he is thinking about opening offices overseas too. If global domination is his ambition, that would make his firm's new office in Gates' neck of the woods especially appropriate.

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