Continuous positive airway pressure (CPAP) is the treatment of choice for obstructive sleep apnoea syndrome (OSAS), but side effects are common and long-term adherence low. The Type D (distressed) personality is defined as a combination of negative affectivity and social inhibition. The association of Type D personality with adherence has not been studied in CPAP-treated patients with OSAS. This study aimed to describe the prevalence of Type D personality in OSAS patients with CPAP treatment longer than 6 months and the association with self-reported side effects and adherence. A cross-sectional descriptive design was used. A total of 247 OSAS patients with a mean use of CPAP treatment for 55 months (6-182 months) were included. Data collection was achieved by two questionnaires; the Type D scale 14 (DS14) (Type D personality), SECI (side effects of CPAP), as well as from medical records (clinical variables and objective adherence to CPAP treatment). Type D personality occurred in 30% of the patients with OSAS and significantly (P < 0.05-0.001) increased the perceived frequency and severity of a broad range of side effects. The objective adherence was significantly lower (P < 0.001) for OSAS patients with Type D compared to OSAS patients without Type D, both with regard to a mean use of 4 h per night and 85% of the self-rated sleep time per night. The additional effect of a Type D personality on perceived side effects and adherence to CPAP treatment found in this study could be used by healthcare personnel when evaluating patients waiting for treatment.