The BIOAIR project focuses on the sub-group of asthma patients that suffer from severe or difficult-to-control asthma, and who have frequent asthma exacerbations ('attacks'). This sub-group of asthma patients consists of around 10% of all asthma patients, but accounts for more than half of the total community costs for asthma. It is as yet unknown why these patients are so much more severely ill compared to the majority of asthma patients that have controlled disease. No adequate treatment has yet been established. The BIOAIR study was initiated in order to find out more about the mechanisms underlying severe asthma. Since some of the changes that occur in severe asthma are similar to patients with chronic obstructive lung disease (COPD), a group of COPD patients was included and will be studied as well. A group with controlled mild-to-moderate asthma was included for comparative reasons.