Wall Street slides at open

NEW YORK (CNNMoney.com) -- U.S. stocks slipped at the start of trading Friday as a Federal Reserve official's comments underlined continued concerns about the nation's economy.

The Dow Jones industrial average lost 0.3 percent. The Nasdaq composite index retreated 0.1 percent. The Standard & Poor's 500 index was 0.3 percent lower.

Among the factors influencing markets at the open:

Fed official speaks: San Francisco Fed President Janet Yellen, speaking in Honolulu, said that while she expects the U.S. economy to avoid a recession, "economic prospects are unusually uncertain."

Stimulus passes: A $170 billion economic stimulus bill passed through Congress late Thursday. The package will send checks of $600 to most taxpayers, and $1,200 to couples, in an effort to boost consumer spending and the economy.

MBIA: The bond insurer said it boosted the size of a public stock offering announced just a day earlier to $1 billion from $750 million in an effort to maintain its AAA credit rating. Stock (MBI)

McDonald's: The world's largest hamburger chain said its same-store sales rose 5.7 percent in January, driven by strong international growth. Stock (MCD, Fortune 500)