This is an article I found on the Mercola website about asthma and Vitamin D. I was interested in it considering my son has asthma. Vitamin D seems to be the most important vitamin to take.
Asthmatic children with low blood vitamin D levels may have a greater risk of suffering severe asthma attacks.
A study followed more than 1,000 children with asthma for four years, and found those with vitamin-D insufficiency at the outset were more likely to have an asthma attack that required a trip to the hospital.
Reuters reports:
“When the researchers considered other factors — including the severity of the children’s asthma at the study’s start, their weight and their family income — vitamin D insufficiency itself was linked to a 50 percent increase in the risk of severe asthma attacks.”

Reuters June 23, 2010
Journal of Allergy and Clinical Immunology June 8, 2010 [Epub ahead of print] Eurekalert June 25, 2010
Eurekalert June 23, 2010
Phytotherapy Research June 14, 2010 [Epub ahead of print]