Pfizer announced last night that it had discontinued research on its most important experimental drug, a treatment for heart disease. The decision is a stunning development that is likely to seriously damage the company’s prospects through the next decades.

Preliminary research found that the drug, torcetrapib, appeared to be linked with deaths and heart problems in the patients who were taking it.

For people with heart disease, Pfizer’s decision to stop the trial represents the failure of a drug that many cardiologists had viewed as a potentially major advance in efforts to reduce heart attacks and strokes.

Torcetrapib is designed to raise levels of so-called good cholesterol. It was to be used in combination with older drugs called statins, like Lipitor and Zocor, which reduce so-called bad cholesterol.

As recently as Thursday, Pfizer executives had hailed the drug at a meeting with investors and analysts at the company’s research center in Groton, Conn.

“This will be one of the most important compounds of our generation,” said Jeffrey B. Kindler, Pfizer’s chief executive.

Pfizer is the world’s biggest drug company, with 106,000 employees and $51 billion in sales in 2005.

In a news release issued yesterday, the company said that it would immediately halt clinical trials of the drug and end its development.

The decision was based on interim results from a 15,000-patient clinical trial. The trial, called Illuminate, was scheduled to be completed in 2009. Pfizer had hoped it would prove that the combination of the two drugs was significantly more likely to reduce heart attacks and strokes than Lipitor alone does.

Even before yesterday’s announcement, some cardiologists had raised concerns about torcetrapib, noting that the drug raised blood pressure in many patients, a serious side effect for a heart medicine. But Pfizer said those concerns would prove to be unfounded, arguing that torcetrapib’s effects on good cholesterol would overwhelm its negative impact on blood pressure.

At this point, it is unclear whether the drug’s failure was due to a specific problem with its chemistry or whether other drugs to raise good cholesterol will also face unexpected problems in clinical trials. Pfizer has other drugs similar to torcetrapib in its pipeline, but they are in much earlier stages of development.

For Pfizer, the end of the torcetrapib program is an enormous blow. Drugs for heart disease are among the most widely used prescription medicines. Lipitor, another Pfizer drug, is the best-selling drug in the world, with sales of $13 billion this year.

The company and investors had expected that torcetrapib would be another big seller, making up for sales that Pfizer will lose when Lipitor loses patent protection in the United States in 2010.

Despite a research budget of $7 billion annually, Pfizer has had difficulty bringing important new drugs to market this decade and its near-term pipeline of new drugs is also thin. On Tuesday, the company said it would cut 20 percent of its American sales force, almost 2,400 employees. Yesterday, it said it intended to accelerate its restructuring because of torcetrapib’s failure.

Pfizer had planned to sell the drug both as a stand-alone drug and in combination with Lipitor. The company had said it expected to file for federal approval in 2007 based on data from other clinical trials that will also be halted.