The results sent shares plummeting more than 30 percent to $2.63 during the trading session, their lowest point in a decade.
The company earned $104 million, or 22 cents per share, after a loss of $59 million, or 12 cents per share, a year earlier. Revenue rose 6 percent to $2.16 billion.
Excluding a $140 million gain from the sale of an investment and other items, the company lost 6 cents per share.
Analysts surveyed by Thomson Reuters expected a loss of 3 cents per share on revenue of $2.21 billion. Those estimates normally exclude one-time gains.
Tenet slashed its 2008 adjusted earnings guidance to $700 million to $750 million for the year from prior guidance of $750 million to $825 million. It also said the 2009 profit guidance of $1 billion will be difficult to achieve, given the weakened economy.