Introduction: CF (cystic fibrosis) and PCD (primary ciliary dyskinesia) are obstructive airway diseases characterised by frequent infections and neutrophilic inflammation. However, PCD has a much milder course than CF. Pilot data showed that in PCD (n=8) the relationship between LCI (lung clearance index) derived from multiple breath washout (MBW), and FEV1 (forced expiratory volume in 1 second) differed from the established correlation in CF. This thesis sought to identify the reasons. Materials and Methods: Larger PCD (n=38) and CF cohorts (n=125), a non-CF bronchiectasis comparator group (n=28), and healthy controls (n=44) were recruited. All performed LCI and spirometry, and subgroups had more complex MBW parameters (conventional and modified phase III analysis and curvilinearity) calculated and HRCT scans scored. Results: As in the pilot data, there was no relationship between LCI and FEV1 in PCD, unlike in CF. PCD patients had fewer structural abnormalities than CF despite similar or worse spirometry and LCI, and the relationship between HRCT and spirometry or LCI in PCD was again different from that seen in CF. MBW analyses showed that Scond* is near-normal in PCD, suggesting less flow asynchrony, compared with CF. Conclusions: There are differences in the nature of distal airway disease between PCD and CF. As the non-CF bronchiectasis patients were similar to CF (rather than PCD), this likely results from the primary mucociliary clearance defect in PCD compared with secondary impairment in the other two conditions. This may be important as care of PCD patients is extrapolated from that of CF patients, which may not be appropriate. It is important not to extrapolate outcome measures uncritically between different disease groups, both clinically and when planning randomised controlled trials. Finally, a better understanding of what causes the better prognosis in PCD may help identify future new treatment avenues in CF.