Patch Adam, whom they made a film of in 1998, seemed to think so (although not very happy with the film they made of him).
But as it's comes under the category holistic care it's hard to prove scientifically, and it often get pushed into the alternative branch with a negative nominator.
I think it's reasonable to assume that it do work for some, as an effect of placebo or not really doesn't matter if it do work for the patient.
It's known that high level of dopamine, serotonin and/or endorphins makes one feel good. One way of producing this is by laughter (ie. from indirect intervention with clowns):
Effects of Laughter on the Human Brain
Edit: (too low score to comment..)
Holistic care only mean (ref. http://medical-dictionary.thefreedictionary.com/holistic+health+care):
a system of comprehensive or total patient care that considers the
physical, emotional, social, economic, and spiritual needs of the
person; his or her response to illness; and the effect of the illness
on the ability to meet self-care needs. Holistic nursing is the modern
nursing practice that expresses this philosophy of care. Also called
This makes it very hard to measure in a scientific test as these require controllable and repeatable factors. The nature, so-to-speak, of holistic care make scientific testing very difficult due to factors influencing the person, that are variable and not controllable. It would be difficult to say if it is the clown, or the financial, or the social etc. that makes the actual effect, and/or which trigger the effect.