(FORTUNE Magazine) – ON MAY 31, Marge Magner was crossing East 71st Street in Manhattan when a cab turning left hurtled into the crosswalk and knocked her down. The impact fractured her left tibia plateau--the bone directly behind the knee that bears most of the body's weight. Surgeons put a plate, three pins, and six bolts into the knee to hold it together. For 14 weeks, Magner used a variety of aids--wheelchair, walker, crutches--to get around. "People had to help me," she says, "and I wasn't used to being helped at all." In August, Magner, the CEO of Citigroup's Consumer Group and No. 5 on FORTUNE's 2004 list of most powerful women, went back to the office--and announced she was leaving.

Research by Catalyst, an organization that studies women in the workplace, shows that attrition rates at the highest corporate levels are comparable for men and women (roughly 10%). But because so few women make it to the top--only 15.7% of corporate officers at FORTUNE 500 companies are women, according to Catalyst--any departure naturally attracts notice. We were curious about why ambitious women would step off the corporate ladder. So we looked up some women whose path had led them onto FORTUNE's power list, then off it--and asked them to tell us their stories.

The backdrop to Magner's departure was a continuing shuffle of Citigroup's top managers. Her mentor at Citigroup, Robert Willumstad, had left the bank in July, and CEO Chuck Prince, who replaced Sandy Weill in 2003, was shaking up Magner's consumer division. "People asked, 'Is this because of Sandy? Is this about Bob Willumstad?'" recalls Magner. "It's not like me to do this, but it's about me. It has nothing to do with them." Magner's mother died this year, and the day the taxi hit her happened to be the last day of shiva, the Jewish mourning period. "Can one's brain ..." Magner said, her voice trailing off. "You can only handle so much loss and so much change and all of that. Life is about everything, not just a certain aspect of it--not just the work." These days Magner is back on her feet and mulling what to do next. She's considering everything from academia to the entertainment industry.

Like Magner, Myrtle Potter (No. 23 in 2004), suffered a life-changing accident. Potter was running commercial operations at the red-hot biotech firm Genentech. In August 2004 she was preparing for a horse show when she approached the final obstacle of the course at the wrong angle, lost her balance, and flew off the horse eight feet into the air before crashing down on her right shoulder, sheering off 70% of the cartilage. (Potter is careful to exonerate the horse: "It was all my fault.")

While Potter was on the mend, a medication to treat the scar tissue in her shoulder triggered a violent allergic reaction that nearly killed her. By mid-January, Potter's liver and kidneys had failed, and her lungs were severely impaired. "I can point to two days when I didn't think I'd make it through the night," Potter says. "Through the course of my recovery, I kept asking myself, 'Have you done all you wanted to do?' The answer for me was no." In August, Potter left her Genentech post.

Her body now healed, Potter still has the energy and drive that propelled her to the top. In addition to being a consultant to Genentech, she's working on behalf of political candidates in California and starting a real estate venture to bring more affordable housing to the Bay Area. One challenge, she says, is finding an assistant who can help juggle all her commitments.

Some women leave the corporate high life but are then lured back. In 1998, Brenda Barnes famously gave up her $2-million-a-year job at PepsiCo to spend more time with her family. Six years later she joined Sara Lee--and now she's CEO (as well as No. 3 on our list). Ann Fudge was the president of a $5 billion unit of Kraft Foods (and No. 34 in 1999) when she decided to take a two-year sabbatical. She came back to corporate life as chairman and CEO of ad giant Young & Rubicam in 2003.

When Carole Black, 61, resigned as CEO of Lifetime TV in March, it was her third exit from a top job. Each time, she says, the men around her have been surprised; some have told her she was crazy to leave so much power at the door. The women, says Black, "totally get it." In 1969 she left her position as a marketing executive at Procter & Gamble to raise her son. In 1993 she quit her job as a senior vice president of marketing at Disney to take a sabbatical and travel the world. Now she's globetrotting again (a kayaking trip in Costa Rica is in the works). Black is not lacking for job offers--this is the woman, after all, who at one point made Lifetime the highest-rated cable network on prime time. But she isn't sure she wants to give up her freedom yet. "I remember reading a line when I was in college. It was from Walden, and it was 'The mass of men lead lives of quiet desperation.' And I got a chill," Black says. "It hit home, and I thought, 'This will never happen to me.' "

Finally, of course, companies reorganize, job responsibilities change, and some women decide it's simply time to move on. Vanessa Castagna, No. 40 on last year's list, could have stayed at J.C. Penney but chose not to after she was passed over for the CEO post. (The board picked outsider Mike Ullman, who had been managing director at the French luxury-goods maker LVMH.) After more than 30 years in retail, though, she couldn't see working for anyone else in the industry. So when she left the company in November, she decided to join Cerberus, a New York-based hedge fund. It turned out to be a smart move. "The only place for me to go was something new like this," she says. "And the timing of the private-equity business is hot. I just hit it."

If there's a single thread that ties together the experiences of these women, it's that taking control of one's own life can feel as bold as wielding power in a corporation. "It's not that they're abandoning it or walking away," Potter says. "I see it as women really exercising their full set of options. And I think that's just a gutsy, powerful thing to do."

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