If you’re a woman looking for some good news about gender parity in business, a quick scan of the Fortune 500 list can make for depressing reading. This year, there was some short-lived cause for celebration.

A total of 32 female CEOs made it onto the 2017 list, the highest proportion of women yet.

These were the lucky few:

CEO Company
Mary Barra General Motors
Ginni Rometty IBM
Indra Nooyi PepsiCo
Marillyn Hewson Lockheed Martin
Meg Whitman Hewlett Packard Enterprise
Safra Catz Oracle
Phebe Novakovic General Dynamics
Irene Rosenfeld Mondelez International
Tricia Griffith Progressive
Lynn Good Duke Energy
Shira Goodman Staples
Geisha Williams PG&E corp
Margaret Keane Synchrony Financial
Deanna Mulligan Guardian Life Insurance Company of America
Barbara Rentler Ross Stores
Debra Crew Reynolds American
Anna Manning Reinsurance Group of America
Vicki Hollub Occidental Petroleum
Debra Reed Sempra Energy
Kathryn Marinello Hertz Global Holdings
Kim Lubel CST Brands
Mary Laschinger Veritiv
Michele Buck Hershey
Patricia Poppe CMS Energy
Denise Morrison Campbell Soup
Kathy Mazzarella Graybar
Sheri McCoy Avon Products
Ilene Gordon Ingredion
Margo Georgiadis Mattel
Beth Mooney KeyCorp
Jacqueline Hinman CH2M Hill
Marissa Mayer Yahoo

True, 32 out of 500 isn’t many. But it was an improvement from last year, when 21 women were at the top of the biggest companies in the US. In 2015, that figure was 24. Research shows that businesses are more profitable when more women are in charge. So it’s not surprising that when the number of female CEOs passed the 30 mark, Fortune giddily announced that women were making “great strides.”

Not so fast.

Already one CEO on the list, former Yahoo chief Marissa Mayer, has been replaced by a man, the previous vice president and chief financial officer of internet company InterActiveCorp, Thomas J. McInerney. He will run what’s left of Yahoo, under the new name Altaba, following the sale of the media business to Verizon on June 13. Long-term Mondelez chief Irene Rosenfeld also will be succeeded by a man: Dirk Van de Put, former chief executive of McCain foods, a Canadian frozen food manufacturer. That leaves 30 female Fortune 500 CEOs.

That number could dwindle further. Kim Lubel, CEO of CST Brands is leaving the company following the completion of its $4.4bn acquisition by Canadian retailer Alimentation Couche-Tard. And this week Sheri McCoy announced that she, too, would be stepping down from the top spot at cosmetics company Avon, (paywall) amid falling shares and pressure from activist investors. It’s possible that their successors will be female. But companies often complain that they can’t find enough women to fill the top spots.

It’s likely that the list of women running Fortune 500 companies will be whittled down once more, to a paltry 28.