(FORTUNE Magazine) – Enron and WorldCom have become America's twin symbols of business malfeasance, but here's a different kind of similarity: In each case the public learned the extent of the scandal in large part through the actions of a brave woman who did the right thing by going over her boss' head.

At Enron, you recall, it was Sherron Watkins who last August sent a long memo to then-CEO Ken Lay, her boss' boss' boss. The rest of the world would spend months trying to untangle Enron's incredibly complex special-purpose entities, but Watkins, an accountant, understood exactly what the problems were and explained them in detail to Lay. She understood that something very bad was going on, something everyone else seemed to think was perfectly okay, and that public revelation would be disastrous: "I realize that we have had a lot of smart people looking at this and a lot of accountants including AA&Co. [Arthur Andersen] have blessed the accounting treatment. None of that will protect Enron if these transactions are ever disclosed in the bright light of day."

That sentence is critical: Watkins was looking at the same facts as plenty of other people inside Enron, yet somehow she was able to escape the groupthink that ensnared her colleagues. Nor did she remain anonymous for long. Soon after writing the memo she identified herself as its author and met with Lay. When her memo eventually became public, everyone understood that Enron wasn't being persecuted; the wrongness of what happened was apparent even internally.

It was a similar story at WorldCom, where another employee accountant saw something that didn't look right and took matters into her own hands. In this case Cynthia Cooper, an internal auditor, began investigating some of the company's capital expenditures in May. This audit had been scheduled for the third quarter, but Cooper got started early; we don't yet know why. That was when she discovered the bookkeeping entries that constituted what looks like the largest accounting fraud in U.S. history.

Faced with disturbing facts, Cooper did exactly what she was supposed to do. She discussed her findings with the company's controller and with Scott Sullivan, the CFO. Sullivan tried to explain to her why costs that had previously been expensed were suddenly being capitalized, starting last year. Then he asked her to stop the audit and put it off until the third quarter.

She didn't.

Instead, she continued--and immediately went over her boss' head and called the chairman of the board's audit committee. He arranged to meet with her and the company's new auditor, KPMG. Two weeks later WorldCom announced it would restate earnings by $3.9 billion--the largest restatement ever.

The importance of Cooper's refusal to postpone her audit, as Sullivan had asked, is even greater than it may appear. Facts uncovered by the company, combined with the memo Sullivan wrote to the board in a last-ditch attempt to defend himself, show that if Cooper had been a good soldier, the whole incredible mess might have been concealed forever.

How do you conceal a $3.9 billion problem? Here's how. Like many companies that made lots of acquisitions, WorldCom was required by a new accounting rule to write down the "impairment" of the assets it had acquired. WorldCom's write-down would be huge--more than $20 billion. Sullivan planned to take it in the second quarter. Also in the second quarter he planned to write down all those improperly capitalized expenses. There aren't many times when a $3.9 billion write-down would look small, but this was one of them. The problem would be gone as of July. Cooper caught it in June.

Is it significant that the whistleblowers in the two great business scandals of recent years were women? It may be. When a business needs shaking up, the best candidate is often an "insider outsider"--someone who has long experience with the company (an insider) but who works away from headquarters or in a noncore division (an outsider); more generally, it's someone who knows where the bodies are buried yet isn't tied deeply to the established powers. I don't know much about the corporate cultures of Enron and WorldCom, but I'm guessing it would be marginally more likely for a woman than for a man to be an insider outsider.

Or maybe my theory is nonsense. Maybe Sherron Watkins and Cynthia Cooper are just two courageous people. It could be as simple as that. The important thing is, as the President, Congress, the SEC, and thousands of companies respond to the trust crisis, let's not forget who revealed quite a bit of it.

GEOFFREY COLVIN, editorial director of FORTUNE, can be reached at gcolvin@fortunemail.com. Watch him on Wall $treet Week With FORTUNE Friday evenings on PBS.