When it comes to succession planning, corporate boards usually prepare for disaster by putting together what they call "the envelope," which contains instructions on who takes over if the chief executive is suddenly incapacitated. Rarely, however, do companies need to pull out these envelopes as frequently as McDonald's recently has. One year ago this week, McDonald's CEO Jim Cantalupo, 60, died of a heart attack, and the board quickly named his deputy, Charlie Bell, to succeed him. Just weeks later, however, Bell was diagnosed with colon cancer. He stayed in the job until November, but by January, Bell, 44, had died, too.

The back-to-back deaths are making corporate boards pay closer attention to the health of the men and women they're considering for top jobs. The goal: to hire healthier CEOs so the envelope never needs to be opened. Recruiter Stephen Mader of Christian & Timbers says many senior executives are already required to have an annual physical. But in reaction to the woes at McDonald's, Mader says, "I have seen insistence on documented evidence" of job candidates' health, such as a note from a physician. Recruiter Gerard Roche of Heidrick & Struggles says that while such requests are still in the minority, they're definitely increasing--and some boards are even asking that CEO wanna-bes be examined by a corporate physician, or making job offers contingent on a satisfactory physical, a hurdle typically faced by free-agent professional athletes.

In the corporate realm, such requests are technically illegal, says employment lawyer Camille Olson of Seyfarth Shaw. Companies, she says, can require pre-employment drug tests but can't demand a physical examination until after they've offered a candidate the job. But since a stock price can tank if a CEO dies suddenly, corporate-governance experts say the due diligence is warranted. "The board needs to say, 'We don't want to invade your privacy, but we're going to need to be informed'," says Nell Minow of the Corporate Library. For a new generation of leaders, that means a history of low cholesterol levels may become as important as a history of high profits.