Kerri Martin is looking for a job. “And I’ve got plenty of time on my hands to do it right,” she declares. Martin–the branding superstar who made a splash reintroducing BMW’s Mini to the United States before switching gears to become director of brand innovation for Volkswagen –doesn’t have much to say about why she abruptly left VW last January: “We’ve agreed not to talk about that.” But she does have some helpful job-hunting tips for her fellow marketing professionals: Know what you’re getting into before you take a gig, interview the firm as intently as it interviews you, and ask yourself, “Can I work with these people?” She pauses, then laughs. “Trust me, I plan to take my own advice.”

There are many others who should too. The chief marketing officer, or its org-chart equivalent, may be the riskiest job in the American C-suite. A veritable who’s who of companies– Coca-Cola , Wal-Mart , Best Buy , Gap , and Verizon –have seen their CMOs depart within the past 12 months. And that’s just déjà vu all over again: For the past three years, an annual survey conducted by executive-search firm SpencerStuart has shown that the tenure for CMOs at the top-100 consumer-branded companies has averaged a scant 23 months. (By contrast, CEOs hang on to their jobs for 54 months.) Some sector averages are particularly grim: If you’re in telecommunications, you’re looking at 15 months; in the food industry, you’ve got about a year.

Greg Welch, author of the survey and an accompanying report, observes, “Clearly, every departure has its own story.” But the new reality is that CMO jobs are incredibly perilous. Today’s marketing chiefs are shape-shifting beasts who grasp not only advertising and promotions but also public relations, IT, finance, manufacturing, customer service, and branding across global markets. “It has more strategic importance than ever before,” Welch says.

That challenge helps explain why “we’re seeing CMOs getting ambushed,” says Donovan Neale-May, executive director of the CMO Council, an international peer-support network whose 3,000-plus members control more than $70 billion in annual marketing spending. One recent casualty was Michael Linton, who was pushed out at Best Buy last year with barely a warning after nearly five years. Linton, now a senior VP at eBay , notes that CMOs are tempting targets. Because marketing is such a public function, and everyone has an opinion on what works, “people will come up to you and say anything, like, ‘Your advertising blows,'” he says. “No one comes up to the CIO and says, ‘Hey, I was just thinking about your data architecture and how much better things are somewhere else.'”

Some marketing honchos do buck the trend. Russell Klein has lasted four years as CMO at Burger King , reenergizing the brand with the creepy but popular “King” character. His relative longevity may have something to do with his background: He brought both agency and beverage-industry experience. But more important, Klein sits comfortably at the helm of a multi-pronged marketing organization that gives him the data–and the relationships–he needs to make the less visible decisions. “Reporting to me are advertising, promotions, and creative, but I’m also on the board of our supply-chain company,” he says. This gives him insights into things such as food and packaging costs. “I can’t just invent a campaign around a new pickle, for example,” he explains. “A company of our size and scale, we’d have to grow that pickle. It’s a new form of produce.” By understanding how small changes might cause inadvertent ripples, he says, he’s able to avoid mistakes.

So what’s it going to take to get the CMO off the endangered-species list? Perhaps a clearer definition of the position and what’s expected–which is a job for the CEO. After all, no company could welcome such regular turnover in a top management position; among other things, it’s inefficient. Indeed, over the past two years, three-quarters of marketing departments have been reorganized, according to a recent Forrester Research report. As Katherine Stone, who survived five CMOs during seven years as director of experiential marketing for Coca-Cola, recently lamented on the blog Brand Autopsy, “Usually, the new CMO gets rid of everything the prior guy championed…. A lot of things get thrown away, which wastes time and money.” Argues the CMO Council’s Neale-May: “CEOs and their boards are having a tough time figuring out what they want.”

Or maybe they aren’t. Maybe the CMO post should be acknowledged simply as the “fall guy” job in the C-suite. If the numbers turn down and CEOs need to make changes, the first instinct certainly won’t be to step aside themselves. Getting rid of the CFO might spook Wall Street, while changing a COO or CIO could disrupt operations. Dumping the CMO seems easy in comparison.