Hanging high over midtown Manhattan, the executive floor of the Lehman Brothers investment bank is a curious fusion of hunting lodge and glass aquarium—only instead of bear pelts and deer antlers, one finds framed announcements of bond offerings on the walls. Lehman's pugnacious C.E.O., Richard Fuld, has his office here, alongside those of the firm's president and chief operating officer, Joseph Gregory, and the company's co-chief administrative officers, Scott Freidheim and Ian Lowitt—men who oversee the army of traders and bankers who make up the capitalist heart of the company. This is not a place where you see many women, unless they're part of the secretarial pool. (View an interactive of key indicators showing gender differences between men and women in corporate America.)

To visit the floor is to realize how remarkable the presence of Erin Callan is. Ensconced in the office next to Gregory's, seated before twin blinking flat screens, Lehman's new chief financial officer is the first woman ever to serve on the firm's 15-member executive committee. But the significance of Callan's promotion doesn't end there. A former tax attorney who started at Lehman in the fixed-income department and then rose to advise hedge fund kings like Ken Griffin and Steve Cohen, Callan led some of the most important initial public offerings in the financial world in recent years, including those for the Blackstone Group and Fortress Investment Group. In the wake of her impressive rise—coupled with shakeups at other Wall Street banks—Callan, 42, may have won a high-stakes game of last-woman-standing. She is possibly the only female now in line to run a major financial institution. (View an interactive timeline of gender wars in America.)

Yet the C.F.O.'s office may be as far as Callan gets. She begins her new job just as women's advances in corporate America and banking in particular have entered a surprising state of limbo. No woman has ever been named the C.E.O. of a Wall Street firm, and the prospects of it ever happening seem even more remote now than they did a year or two ago. The most promising female candidate, Morgan Stanley co-president Zoe Cruz, was pushed out last fall after the firm posted a $3.7 billion loss from mortgage-related securities. Merrill Lynch, Bear Stearns, and Citigroup underwent C.E.O. searches in recent months, and no women appeared on any of their short lists. (Read more about the workplace battle of the sexes.)

A handful of women have achieved C.F.O. status at major Wall Street firms. The select group includes Sallie Krawcheck (briefly) and Heidi Miller before her at Citigroup, as well as Dina Dublon at J.P. Morgan Chase and Barbara Yastine at Credit Suisse First Boston. None of them have moved to the top job at their firm or even to an obvious precursor position. Miller went on to become C.F.O. of Priceline and later Bank One; she's now in charge of J.P. Morgan Chase's treasury and security services business. Dublon retired in 2004, at age 51, after serving as C.F.O. for six years, and Yastine left her C.F.O. slot to devote herself to philanthropy. Krawcheck, whose coronation and sudden move sideways out of the C.F.O. office were documented breathlessly by the business press, is now Citigroup's head of global wealth management and no longer considered a contender for the top job. (Sexism or Not? You Decide)