The study finds simply anticipating a mirthful experience reduces potentially detrimental immune system stress hormones such as cortisol by 39 percent, epinephrine by 70 percent and dopac by 38 percent.

The researchers used a protocol similar to one used two years earlier to find an increase in two "good" hormones -- beta-endorphins that alleviates depression increased by 27 percent and human growth hormone, which helps with immunity, increased by 87 percent -- when volunteers anticipated watching a humorous video. There was no such increase among the control group who did not anticipate watching the humor film.

"Our findings lead us to believe that by seeking out positive experiences that make us laugh we can do a lot with our physiology to stay well," study lead researcher Dr. Lee Berk of the Oak Crest Health Research Institute, in Loma Linda, Calif., said in a statement.

The findings are being presented at the 121st annual meeting of the American Physiological Society -- part of the Experimental Biology 2008 scientific conference in San Diego.