Continuous positive airway pressure (CPAP) is the treatment of choice for obstructive sleep apnoea syndrome (OSAS), but many patients find this treatment intolerable. The aim of this study was to characterize patients who were unable to tolerate CPAP treatment (non-complaint) as opposed to those who continued using CPAP (complaint). A case-control study was performed in which the cases comprised of 40 patients who had been started on CPAP treatment but had found the treatment unacceptable and had ceased to use CPAP. The controls comprised of 63 patients with OSAS who had been prescribed CPAP and were still using it (follow-up period 18 months to 10 yr). The patients who stopped CPAP treatment had a higher mean age, had more frequently undergone uvulopalatopharyngoplasty (UPPP) and had a lower mean oxygen desaturation index (ODI) than patients who continued using CPAP. ODI was an independent negative predictor of non-compliance (OR5units=0.6(0.4-0.8), P<0.01). The two most common reasons for non-compliance were problems in the nose or pharynx and lack of subjective effect by the treatment. High age was an independent risk factor for non-compliance because of problems in the nose or pharynx (OR10years=2.8(1.3-6.1), P<0.01), while having undergone UPPP was a risk factor for non-compliance because of lack of effect (OR=4.5 (1.1-19.1), P<0.05). In conclusion, patients with less severe OSAS are more likely to discontinue CPAP treatment. The risk of experiencing nasal and pharyngeal side-effects of such severity that the patient stops using CPAP increases with age and patients who have undergone UPPP are less likely to experience a clinical improvement after being started on CPAP therapy.