It is well documented that there are significant ventilatory changes in respiratory diseases such as cystic fibrosis, asthma and the onset of bronchiolitis obliterans syndrome (BOS) following chronic rejection of lung transplantation. At present, we use measures such as spirometry and lung biopsies to determine the changes of airway function and disease severity. Our aim is to develop a novel technique called the multi breath nitrogen washout (MBNW) which we believe is able to measure the inhomogeneity of ventilation in both the larger airways (conductive region, generation 1 - 16) as well as the smaller airways (acinar region 17 - 23). Our belief is that these measurements are much more subtle than current techniques and will be more sensitive in measuring large and small airway changes in disease. The MBNW can also give us an insight as to which particular zones of the lung are affected in differing respiratory disease. For example, it is believed that BOS begins at the distal portion of the lung (acinar region) and proceeds towards the proximal zone (conductive). However, at present no current techniques can differentiate between damage to the acinar zone and the conductive zone or indeed accurately measure small airway (acinar zone) function. We believe the the MBNW has the capacity to do so.