To be sure, that sum represents a disproportionately small share of the group as a whole; just 6.6%. But it also marks a considerable jump from last year’s total of 24, or 4.8%.

The uptick in female CEOs this year is largely the result of women being named chief executive in the last 12 months, such as Best Buy’s Corie Barry, Northrop Grumman’s Kathy Warden, and Land O’Lakes’ Beth Ford. In fact, the 33rd CEO to make the list did so just this week, when home goods retailer Bed Bath & Beyond tapped Mary Winston as its interim CEO amid pressure from a trio of activist investors.

Two of the new additions—Williams-Sonoma CEO Laura Alber and Advanced Micro Devices CEO Lisa Su—have held their jobs for years, since 2010 and 2014, respectively. They’re joining the club now because their companies have surpassed the Fortune 500’s revenue cut-off of $5.575 billion.

The relative barrage of new female CEOs runs counter to last year’s narrative, when the departure of high-profile chief executives—Hewlett Packard Enterprise’s Meg Whitman, Mondelez’s Irene Rosenfeld, Campbell Soup’s Denise Morrison—pushed the count down to 24.

The number of female CEOs in the Fortune 500 is, of course, vulnerable to the whims of individual companies, but, as a whole, it can also be seen as a barometer of women’s standing in the business world.

So what might be behind the net increase of nine in one year’s time? In a word: boards.

“We are seeing women and minorities on boards ticking up, and boards have a lot to do with who becomes CEOs,” says Lorraine Hariton, CEO of Catalyst, a nonprofit consulting and research firm focused on women in the workplace. Fifteen years ago, women accounted for 15.7% of board seats in the Fortune 500. Now, it’s 25.5%. The increase has come as institutional investors—citing research on the business benefits of diverse leadership—have pushed for new blood in boardrooms.

Christy Glass, a professor at Utah State University who focuses on gender inequality and race and ethnicity in work and leadership, says her research with co-author Alison Cook “has shown that when boards are well-integrated with women, women are much more likely to be appointed CEOs.” The push for board diversity, she says, “may be paying off in terms of women appointed as CEOs.”

What’s more, her research has found that having a gender-diverse board also increases the likelihood that women CEOs have lengthier tenures; women CEOs in the Fortune 500 overall have shorter stints than their male counterparts—42 months versus 60.

While the new Fortune 500 list marks a record for total number of female CEOs, it also continues a troubling trend: that so few of those chief executives are non-white women. Last year’s tally included PepsiCo’s Indra Nooyi, who’s Indian-American, and PG&E CEO Geisha Williams, who’s Latina. Both left their posts in recent months, as Nooyi retired, and Williams departed amid PG&E’s wildfire crisis. Bed Bath & Beyond’s Mary Winston is the first black woman to serve as a Fortune 500 CEO since Xerox’s Ursula Burns stepped down two-and-a-half years ago, although Winston is in an interim post.

Down the line, this problem may be helped by the board diversity push too, Glass says. “Our research suggests that when women lead companies at the board rank and as CEOs, [there’s] more attention [paid] to equality policies and practices. So one added bonus of the growth of women directors is that CEOs place more emphasis on recruiting, retaining, and advancing people of color.”

More must-read stories from Fortune:

—The 2019 Fortune 500 list demonstrates the prize of size

—Why the giants among this year’s Fortune 500 should intimidate you

—The Occidental-Anadarko merger reveals the crude truth about oil prices

—It’s all clicking for Wayfair, a Fortune 500 newcomer

—Fortune 500 CEO Survey: The results are in

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This story has been updated to correct the x axis of the female CEOs chart.