"We believe laughing is good for your health," said Michael Miller of the University of Maryland School of Medicine in Baltimore, who led the research. "And we think we have evidence to show why that's the case."

A growing body of other evidence has suggested that negative emotions, particularly depression and stress, can be harmful, making people more prone to illness, more likely to experience suffering from their ailments and less likely to recover as quickly, or at all. One recent study even found sudden emotional shock can trigger life-threatening heart symptoms that many doctors mistake for a classic heart attack. Miller himself, along with his colleagues, had done a study that found people who have a negative reaction to social situations tend to be more prone to heart disease.

But far less has been done to examine whether positive emotions can reduce the risk and complications of illness.

"The focus is always on the negative aspects," he said. "We thought, 'Why not look at the opposite?' "

So they decided to examine the ability of blood vessels to expand -- known as vasodilation. Poor vasodilation can increase the risk of heart attacks and strokes by making the passageways prone to being blocked, cutting off vital blood flow.

The researchers asked 20 healthy men and women to watch clips of two movies -- either the violent opening battle scene in the 1998 film "Saving Private Ryan" or a humorous scene from a comedy, such as the 1996 "Kingpin."

The researchers tested the subjects' vasodilation, before and after the movie, by constricting and releasing an artery in their arms with a blood pressure cuff and then using ultrasound to measure how the blood vessels were functioning.

The researchers discovered striking differences depending on which movie the volunteers had watched. Blood flow was significantly reduced in 14 of the 20 volunteers who saw the stressful film. In contrast, blood flow markedly increased in 19 of the 20 volunteers after watching the funny movie, the researchers reported last week at a meeting of the American College of Cardiology in Orlando.

Overall, blood flow decreased by about 35 percent after experiencing stress but increased 22 percent after laughter -- an improvement equivalent to that produced by a 15- to 30-minute workout.

"It was a pretty dramatic difference," Miller said.

Previous research has indicated that stress hormones may be the primary culprit by which negative emotions harm health. When a person is under stress, the body pumps out hormones such as adrenalin and cortisol. That is designed to prime the body for a fight or a flight, but the hormones can have detrimental effects on the body, including suppressing the immune system and constricting blood vessels.

Miller and his colleagues hypothesize that laughter may have a contrasting effect, causing the body to release other natural chemicals known as endorphins -- pleasure-producing agents best known for producing the "runner's high" -- that may counteract the effects of stress hormones and cause blood vessels to dilate.

The researchers acknowledge they are still a long way from proving their hypothesis or fully understanding the process, but they say the theory makes sense.