Got a mentor? Good. Now find a sponsor.

September 21, 2012: 10:40 AM ET

Mentoring, a hot topic when our first Most Powerful Women list launched in 1998, has helped plenty of women get ahead. Sponsorship can help even more -- but it's harder to come by.

FORTUNE -- Bridget Van Kralingen, senior vice president of global business services at IBM (and No. 23 on our list of Most Powerful Women), tells two stories, from different points in her career, that illustrate the difference between a mentor and a sponsor.

When she first joined Deloitte Consulting, she also moved from her native South Africa to the U.S. "A young partner in New York sat me down and said, 'Bridget, you have tremendous potential, but you're way too nice and polite.' That was great mentoring!" she says. "It helped me understand the cultural differences. I quickly adapted." She eventually became managing partner of Deloitte's financial services business.

Then, in 2004, Van Kralingen joined IBM (IBM), which has included mentoring as part of its management training since Thomas Watson, Sr. founded the company in 1916. CEO Ginni Rometty, who tops our list, has mentored dozens of women and men, both formally and informally, and credits mentors for several key promotions in her own career.

At Big Blue, Van Kralingen recalls, "I had a sponsor who recommended me to run the financial services part of IBM's business consulting services in Europe and Africa -- in spite of my lack of IBM experience. His advocacy both helped IBM to take a risk on me and helped me navigate the job."

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Mentoring, in other words, prepares people to move up, while sponsorship makes it happen. Both are "critically important," says Van Kralingen, to anyone aspiring to get ahead. The not-so-great news: Most big companies now have embraced mentoring and, in the high-potential pipeline that leads to senior management, more women than men have had multiple mentors. Yet the percentage of senior executives who are female has barely budged since 1998, creeping up from 11.2% then to 15.7% now, according to nonprofit research group Catalyst.

A big reason why that's so, according to one Catalyst study of 4,000 MBAs of both sexes, is that men are still more likely than women to have powerful sponsors. "High-potential women are overmentored and undersponsored, relative to their male peers," observes Christine Silva, a Catalyst senior director. "Without sponsorship, women not only are less likely than men to be appointed to top roles, but may also be more reluctant to go for them."

Because it requires a senior executive to spend his or her own political capital, and put his or her own credibility on the line, to give an underling a leg up, sponsoring someone is far riskier than mentoring them. "A sponsor is someone influential who will pound the table for you," notes Claire Farley, who appeared on our 1998 list as head of North American oil and gas exploration at Texaco (since merged with Chevron (CVX)) -- a job she took on while still in her 30s, thanks partly to a high-ranking sponsor. She's now a managing director at private-equity powerhouse KKR (KKR) in Houston. More

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