Continuous positive airway pressure (CPAP) is the treatment of choice for obstructive sleep apnoea syndrome (OSAS), but low adherence rates are common. The aim was to develop the attitudes to CPAP treatment inventory (ACTI), and to investigate the validity and reliability of the instrument among patients with OSAS. ACTI was developed on the basis of: (i) in-depth interviews with 23 patients; (ii) examination of the scientific literature; and (iii) consensus agreement of a multi-professional expert panel. This yielded five different types of attitudes to CPAP treatment. A prospective longitudinal design was used. Two-hundred and eighty-nine patients with OSAS were recruited at three different CPAP centres. Data were collected with ACTI and obtained from medical records. The homogeneity and internal consistency reliability were satisfactorily reflected by the item-total correlations (0.59-0.81) and Cronbach's alpha (0.89), respectively. Construct validity was confirmed with factor analysis (principal component analysis with orthogonal rotation; PCF). The PCF based on baseline data resulted in a one single-factor solution explaining 69% of the total variance. A confirmatory factor analysis was performed 2 weeks after CPAP initiation, resulting in the same factor solution. No indication of uniform differential item functioning was found. The predictive validity was tested with receiver operating characteristic analyses, and a cut-off of 10 on the ACTI gave a sensitivity of 93% and a specificity of 44% for CPAP termination within 6 months. The satisfactory measurement properties of this new pragmatic instrument are promising and indicate that ACTI can be useful in clinical practice to reliably measure attitudes to CPAP treatment.