Shares of Chipotle Mexican Grill Inc.(CMG), a restaurant chain spun off by McDonald's Corp. (MCD), surged to more than double their $22 initial public offering price on Thursday.

Chipotle shares rose to $46.20 on the New York Stock Exchange, up $24.20.

But as the fast-growing chain became the first runaway U.S. IPO of 2005, analysts warned that dramatic first-day surges in stock price are often followed by declines.

"Is it going to $53.25 or $33.25 in the next move? I would say it would be to the downside, and rightly so," said David Menlow, president of IPOFinancial.com, an independent research firm based in Millburn, N.J.

"We like the company, we like what they do, everything about it is great, but at some point you can't breathe too well at 43,000 feet," Menlow added.

At the midpoint of its first forecast price range of $15.50 to $17.50 per share, analysts calculated Chipotle shares would have had a historic price-to-earnings ratio of 28.7, slightly higher than its most comparable competitor, Rubio's Restaurants Inc. (RUBO). Based on Thursday's prices, Chipotle's price-to-earnings ratio would be triple Rubio's.

One cautionary example is shares of Chinese Web search company Baidu.com Inc. (BIDU), which more than quadrupled in their August Nasdaq debut, trading as high as $151.21, but have since eased back to about one-third that value.

The 7.9 million-share Chipotle offer was worth $173.8 million.

Steven Ells, Denver-based Chipotle's chairman, chief executive officer and founder, said the chain planned to put the bulk of the proceeds from the IPO toward funding expansion, with plans to open 80 to 90 stores this year.

The company plans to expand into Boston and Philadelphia late this year or early 2007 with likely only a handful of stores at first, he told Reuters.

"Most of our growth will be in our existing markets," Ells said, citing Washington, D.C., Minneapolis, Chicago and many California cities as the company's core growth area.

Chipotle sold 6.1 million shares and McDonald's offered 1.8 million, according to a U.S. Securities and Exchange Commission filing.

McDonald's, the world's largest fast-food company, remains a majority holder of the 489-store "fast-casual" restaurant chain.

No. 3 U.S. burger outlet Wendy's International Inc. (WEN) is planning an IPO for its Tim Hortons unit later this year.

Morgan Stanley and S.G. Cowen & Co., a unit of Societe Generale, were the lead underwriters.