This year's Fortune 500 has made history with the female 18.

The 2012 ranking of the 500 largest corporations in the United States includes a record 18 firms helmed by female CEOs, up from 12 companies in 2011.

The previous record for women-led companies in the Fortune 500 was set in 2009, and included 15 firms run by female executives. Just seven Fortune 500 companies had female CEOs in 2002 and 2003.

Though this year marks a new high for female CEOs, women still run just 3.6 percent of Fortune 500 companies. And one in 10 Fortune 500 corporations have no women on their boards, according to research by Catalyst, a non-profit organization that seeks to expand women's roles in the workplace.

Fortune executive editor Stephanie Mehta said she expects the ratio to continue its shift given the growing number of high-level female executives at U.S. corporations.

“The good news is that while we have 18 today, there’s a pipeline of women coming into leadership positions that’s very, very deep and very, very wide,” Mehta said. “There are women sitting just below the CEO position at these Fortune 500 companies and many of them are poised to lead Fortune 500 companies when there are openings and movement.”

Though Silicon Valley is often described as a boys’ club where female CEOs are few and far between, the two largest Fortune 500 corporations run by women are both in the tech industry: Hewlett-Packard, which ranked 10th on Fortune’s list and named Meg Whitman CEO last fall, and IBM, which ranked 19th and has been run by Ginni Rometty since January.

Other female chief executives in the Fortune 500 include Archer Daniels Midland CEO Patricia Woertz, Pepsico CEO Indra Nooyi, Kraft Foods CEO Irene Rosenfeld, Xerox CEO Ursula Burns and Avon CEO Sherilyn McCoy. (Fortune has a list of the 18 women here, which includes their career advice and reflections on the glass ceiling).

But don’t expect to see parity between the sexes on the Fortune 500 anytime soon, cautions Mehta, who argues that an even ratio on the Fortune ranking should not be the benchmark for equality in the workplace.

“I don’t think we'll get to 50/50 in the near future, but I’m not necessarily sure that that’s the goal. Running a Fortune 500 company is just one of the options available to strong women in leadership positions,” Mehta said. “We may see a growing number of women take the choice to become leaders in non-Fortune 500 companies. And when we have choices and when women are in control of their careers and decision making in their careers, that’s true equality. It doesn’t necessarily need to be measured in number like 50/50.”