Anticipation of laughter reduces the levels of stress hormones:
http://www.sciencedaily.com/releases/2008/04/080407114617.htm 

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.
http://www.holisticonline.com/Humor_Therapy/humor_therapy_benefits.htm

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.
http://www.umm.edu/news/releases/laughter2.htm 

Average blood flow increased 22% during laughter, and decreased 35% during mental stress. 
http://www.abc.net.au/science/news/stories/s1318660.htm 

Blood glucose levels, a key gauge for development of diabetes, became lower after participants laughed compared with after listening to a lecture.
http://care.diabetesjournals.org/cgi/content/full/26/5/1651 

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.)
http://linkinghub.elsevier.com/retrieve/pii/S0022399907000293 

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
http://www.mendeley.com/research/laughter-abolishes-binocular-rivalry/
http://www.abc.net.au/science/news/stories/s1294404.htm

Workplace Laughter Groups positively affect personal efficacy beliefs, such as self-regulation, optimism, positive emotions and social identification.
http://www.bellylaughday.com/images/laughterlinksAuthors_20Full_20Text.pdf 

Reminiscing about laughter makes you laugh again and improves relationship satisfaction. 
http://theapp.appstate.edu/content/view/1845/42/ 

Employees who start the day in a good mood tend to stay that way.
http://www.upenn.edu/researchatpenn/article.php?1101&bus

WebMD Heartwire article discussing the first study of laughter and blood pressure using Laughter Yoga. 
http://www.medscape.com/viewarticle/574552 
Results were published in May 2008 in The Journal of Clinical Hypertension Supplement A, Volume 10, Issue 5.

Laughter and palliative care.
http://www.ncbi.nlm.nih.gov/pubmed/2713108?dopt=Abstract
http://www.informaworld.com/smpp/content~content=a785035256~db=all

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.
http://www.hohohahaha.com/Laughter_Yoga_for_Schizophrenia_&_Bipolar_Depression.pdf

 

     
 
Research