Rosalind "Roz" Brewer is about to become only the second Black woman to permanently lead a Fortune 500 company. She starts as Walgreens CEO on March 15.

Why it matters: It's a stark reminder of how far corporate America's top decision-makers have to go during an unprecedented push by politicians, employees and even a stock exchange to diversify their top ranks.

• The former Starbucks COO, who played a key role in shaping the company's pandemic response, is the cohort's first Black woman in a permanent CEO job since Ursula Burns stepped down from Xerox in 2016.
• Mary Winston served as the interim CEO of Bed Bath & Beyond for six months in 2019 — until Mark Tritton, a white man, was named permanent CEO.

The big picture: Diversity among Fortune 500 chief executives across the board is a disappointment. The number of Black Fortune 500 CEOs peaked at six in 2012. Since then, it's gotten worse.

• Brewer will now be one of five Black Fortune 500 chief executives — alongside Merck's Kenneth Frazier, Lowe's Marvin Ellison, M&T Bank's René Jones, and TIAA's Roger Ferguson.
• That will slip to four in March — when Ferguson is set to leave his post.

Between the lines: While the number of women heading the country's biggest companies is still unimpressive, there are currently more than ever before (41).

• The list is growing: Walgreens rival CVS named Karen Lynch as its new CEO last year. She takes the helm on Feb. 1. Jane Fraser takes over as Citigroup's first woman CEO next month.

What's next: If America's largest companies want to pull from within their top ranks, potential Black successors aren't plentiful.

• Take the 100 of the biggest U.S. companies: As of last year, just 1% of CFOs and 4% of corporate division heads at 100 of America’s largest firms are Black, per research by Stanford University.