Background: Obstructive sleep apnoea-hypopnoea syndrome (OSAHS) carries substantial negative health consequences. This study examines factors affecting mortality and morbidity according to continuous positive airway pressure (CPAP) use and predictors affecting CPAP adherence in a longitudinal cohort of OSAHS patients., Materials and Methods: This prospective, cohort study comprised 4502 patients who were diagnosed with OSAHS at a tertiary sleep disorders centre between 1982 and 2003. Of these, 1174 patients completed follow-up in 2012. Data collected included anthropometric, sleep and demographic characteristics, including comorbidities, ongoing medications and CPAP adherence. Patients were followed up for an average of 14.8±3.7 years., Results: Imputation analysis showed that long-term CPAP users (>5 years) were 5.63 times more likely to be alive at study end than non-CPAP users (95% CI: 4.83-6.58, p<0.001) and 1.74-times more likely than short-term CPAP users (≤5 years) (95% CI: 1.49-2.02, p<0.001). Females had a significantly higher mortality rate during the follow-up period (26.8% versus 19.6%, p<0.001). Respiratory mortality was more common in patients with OSAHS, in particular those who did not use CPAP, compared to the general population (17.2% versus 12.2%, p=0.002 respectively), whereas deaths from cancer were less common compared to the general population (16.2% versus 25.6%, p<0.001). Compared to CPAP users, non-CPAP-users had a significantly increased incidence of type II diabetes mellitus (DMII) (27.9% versus 18.7%, p=0.003), ischaemic heart disease (IHD) (25.5% versus 12.7%, p<0.001) and myocardial infarction (MI) (14.7% versus 4.2%, p<0.001) at long-term follow-up., Conclusions: Long-term CPAP use in men and women with OSAHS reduces mortality and decreases the incidence of DMII and cardiovascular disease., Competing Interests: Conflict of interest: S. Dodds has nothing to disclose. Conflict of interest: L.J. Williams has nothing to disclose. Conflict of interest: A. Roguski reports that her PhD has been jointly sponsored by the BBSRC and Eli Lilly & Co. since October 2018, outside the submitted work. Conflict of interest: M. Vennelle has nothing to disclose. Conflict of interest: N.J. Douglas reports that he is a stockholder in ResMed, outside the submitted work. Conflict of interest: S-C. Kotoulas has nothing to disclose. Conflict of interest: R.L. Riha has nothing to disclose., (Copyright ©ERS 2020.)