Asthma is a complex, heritable disease that affects more than 11.2% of the U.S. population, which represents approximately 9 million children and 23 million adults. It is thought that asthma may be caused by the interaction of multiple genetic factors, such as whether a parent or sibling has asthma, and environmental factors, such as living in an inner city. Studying DNA from people who have asthma and those who do not have asthma may help us better understand the importance of those genes and how they are involved in asthma severity and response to treatment.