Anticipation of laughter reduces the levels of stress hormones: 

Boosts immune function by raising levels of infection-fighting T-cells, disease-fighting proteins called Gamma-interferon and B-cells, which produce disease-destroying antibodies. Triggers the release of endorphins, the body's natural painkillers, and produces a general sense of well-being.

Laughter causes the endothelium, the inner lining of blood vessels, to dilate. This increases blood flow which, of course, is good for overall cardiovascular health. 

Average blood flow increased 22% during laughter, and decreased 35% during mental stress. 

Blood glucose levels, a key gauge for development of diabetes, became lower after participants laughed compared with after listening to a lecture. 

Laughter modulates prorenin receptor gene expression in patients with type 2 diabetes. Journal of Psychosomatic Research. (Please note: This article is now pay to view.) 

Laughter can literally change your perceptions: Professor Jack Pettigrew reports his findings on how laughter affects visual perception in the journal Clinical and Experimental Optometry

Workplace Laughter Groups positively affect personal efficacy beliefs, such as self-regulation, optimism, positive emotions and social identification. 

Reminiscing about laughter makes you laugh again and improves relationship satisfaction. 

Employees who start the day in a good mood tend to stay that way.

WebMD Heartwire article discussing the first study of laughter and blood pressure using Laughter Yoga. 
Results were published in May 2008 in The Journal of Clinical Hypertension Supplement A, Volume 10, Issue 5.

Laughter and palliative care.

Not a scientific study, but a write up from a colleague in Australia who has used Laughter Yoga with mentally ill patients at a local facility.