Accel now has $4 billion under management since it was established in 1983. Palo Alto, Calif.-based Accel plans to invest the $520 million from Fund X over roughly three years, concentrating on early-stage information technology companies.

Theresia Gouw Ranzetta and Rich Wong, recent additions to Accel's partnership, said the company will invest the new vehicle in keeping with its existing strategy of "focusing on early-stage venture investing in our core sectors: consumer Internet, digital media, software, mobile and networking, and infrastructure."

Accel makes being the first, or one of the first, of a startup's backers a key part of its investment strategy, since getting in early on its portfolio companies can help net a larger stake at a lower valuation. The downside of an early-stage focus can be a higher number of portfolio companies that wash out and never gain enough traction to establish themselves.

The new vehicle is 30% larger than its predecessor, Accel IX, which closed in 2004 with $400 million and is still being deployed.

Ranzetta and Wong said, "Accel X is slightly larger than Accel IX was because we have a larger investment team of nine partners and two new partners. We also believe that conditions are positive for both consumer and enterprise segments for the next four years."

While the size of Accel IX was itself scaled back from Accel VIII, which closed with $950 million in 2000, the IRR of both vehicles is likely among the industry's top echelon thanks to a string of exits -- all above $100 million -- from both vehicles over the past 12 months.

In the last 12 months, portfolio companies Riverbed Technology Inc., MetroPCS Communications Inc., comScore Inc. and Infinera Corp. have all had successful initial public offerings. Accel was a Series A investor in all four. On the M&A side this year, XenSource was bought for $500 million, Zimbra Inc. was acquired for $350 million, Acopia Networks Inc. fetched $210 million, and Reactivity Inc. sold for $135 million.

While the string of exits has given Accel's prior funds a big boost in their IRRs, the biggest exit of all -- Facebook Inc. -- waits in the wings. Accel and PayPal Inc. founder Peter Thiel were some of the earliest investors in the social networking powerhouse and have participated in all three of the company's venture rounds.

When Accel put $13 million into Facebook's first round in April 2005, reports placed Facebook's valuation at $100 million. Two and a half years later, Facebook's valuation stood at $15 billion after Microsoft Corp. paid $240 million for a 1.6% of the Palo Alto, Calif., company.

While the size of Accel's stake has not been disclosed, its value is almost certainly measured in billions of dollars.

Accel X is the second venture fund that the firm has closed in 2007.