Steven Cohen

By Justin Fox

Fifteen years ago, options trader Steven A. Cohen left the middling Wall Street firm where he worked to start a hedge fund. Hardly anybody noticed, and Cohen appeared to like it that way. He spent the next decade quietly earning spectacular returns (average annual gain of nearly 40%, even after Cohen's hefty fee) by making trade after hair-trigger stock and options trade—and discussing them with almost no one.

Eventually, though, Cohen became so successful that he could no longer fly under the radar. Forbes last year estimated his net worth at $3 billion. The huge addition he built onto his house (plus ice rink) became the talk of Greenwich, Conn. And because of its huge trading volumes and reputation for moving stock prices, Cohen's hedge fund, SAC Capital, with 700 employees and $13 billion under management, has become perhaps the most talked-about force on Wall Street. Cohen, 50, the son of a dress manufacturer from Long Island, N.Y., has adjusted, in a fashion. His firm now holds on to stocks longer and has branched into lending and private equity. He has become a major collector of modern art. Last year he even spoke to the Wall Street Journal! A couple of things haven't changed, though: Cohen still spends weekdays glued to his trading desk, and in 2006 the firm's main fund was up an ad