In any case, contrary to the impression given above there are no peer-reviewed medical studies proving that eating asparagus alone "prevents" or "cures" cancer. That's not to say asparagus offers no cancer-fighting benefits whatsoever — there's a good chance it does, given that it contains vitamin D, folic acid, and the antioxidant glutathione, all thought to play some role in lowering risk factors for certain cancers. By all means, eat your asparagus!
The thing is, lots of other foods provide the same nutritional benefits and more besides, so emphasizing one particular vegetable over all the other health-promoting foods available is surely counter-productive. Generally speaking, medical experts recommend a diet high in fiber, fruits and vegetables and low in fats and nitrates for optimal resistance to cancer.
At the risk of stating the obvious, it should also be noted that dietary measures ought never to be regarded as a substitute for proper medical diagnosis and treatment of any disease, especially cancer.
See also: Lemons and Cancer
Sources and further reading:
Diet and Disease
ADAM Health Encyclopedia, 8 August 2007
Colorado Dept. of Public Health and Environment
Looking for Health Benefits? Try Asparagus
The Telegraph, 22 April 2009
A Few Words in Favor of Asparagus
Britannica Blog, 4 December 2008
Top Cancer-Fighting Foods
WebMD.com, 24 April 2006
Last updated: 04/29/09