Chronic rejection, more commonly called bronchiolitis obliterans syndrome (BOS), is the leading cause of death beyond the first year post lung transplantation. The diagnosis of BOS is typically made by clinical, physiological, and radiographic parameters. Early detection would be desirable since it allows treatment modification to stop or delay the process. In the last few years there has been a growing interest in lung clearance index (LCI), a measure of lung physiology derived from multiple breath washout tests. LCI is derived from Multiple Breath Washout (MBW) tests. Early detection of BOS with LCI measurement will allow the investigators early recognition of this chronic rejection form and with early institution of the enhanced treatment survival will increase. All paticipants who underwent bilateral lung transplantation at Zurich University Hospital will be included. The measurement will be done 3 months after lung transplantation. Approximately 90 paticipants will be included. The follow-up will be 5 years. If the investigators could detect the development of BOS with this novel method before the clinical deterioration (fall in lung function) the investigators can start the available treatment options before irreversible damage occurs. This might increase overall survival in the study cohort.