Objectives: In daily practice, we encounter with obstructive sleep apnoea syndrome (OSAS) patients who require different levels of positive airway pressure (PAP) despite having a similar apnoea-hypopnea index (AHI). We aimed to determine the parameters contributing to the determination of the therapeutic level of PAP. Methods: Data on 548 patients who underwent polysomnography and PAP titration were analysed retrospectively. Patients were divided into groups according to OSAS severity (mild, moderate, and severe) and the mean pressure in each group was determined, after which patients were further divided into those who required a PAP below the mean and those who required a PAP above the mean. Results: The mean optimal PAP level in the mild, moderate, and severe OSAS groups was 7.4 ± 2.3, 8.6 ± 2.4, and 9.8 ± 2.9 cm H 2 O, respectively. In the moderate and severe OSAS group, the subgroup that needed high pressure had a higher supine AHI, a longer apnoea time, and a longer SaO 2 <90% time as compared with the subgroup that needed low pressure. Conclusion: A longer apnoea duration and a higher supine AHI are associated with a higher PAP level in patients with moderate and severe OSAS.