What: Shares of diet specialist Weight Watchers (NYSE: WTW) were looking slimmer today, as they lost as much as 10% in intraday trading.

So what: There's not a whole lot more that Weight Watchers could have done. Before the open today, the company announced first-quarter earnings, and the numbers looked darn good. Revenue of $503 million was up 30% from last year and was ahead of the $478 million that analysts were looking for. On the bottom line, the company's $1.00 in earnings per share was 73% better than the first quarter of 2010 and easily topped Wall Street's $0.89 target. In addition, total paid weeks grew 40% on a strong showing from the online segment.

Now what: Wait, it gets better! The company also raised full-year earnings guidance to a range of $3.75 to $4.00 from a range of $3.50 to $3.85. That kind of move almost never fails to get investors excited. Yet in Weight Watchers' case, it did. What gives?

Well, Weight Watchers' stock has been binging on gains over the past year, bulking up by around 170%. Currently the stock trades at roughly 18 times the midpoint of management's new 2011 guidance. That isn't a particularly egregious multiple, but investors sitting on big gains from the past 12 months may be getting a little antsy.

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Fool contributor Matt Koppenheffer has no financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool’s disclosure policy prefers dividends over a sharp stick in the eye.