The Securities and Exchange Commission has begun a broad examination into whether Wall Street bank employees are leaking information about big trades to favored clients, like hedge funds, in an effort to curry favor with those clients, executives at Wall Street banks said.

The inquiry, these people said, seems aimed at determining how pervasive insider trading, or the illegal use of market-moving nonpublic information, may be on Wall Street. Knowledge about a large trade, like the sale of a big block of stock by the mutual fund giant Fidelity, would tell a trader which way the stock would move.