BACKGROUND AND OBJECTIVES: The aim of this study was to determine the relationship between the continuous positive airway pressure (CPAP) titration level and upper airway variables in obstructive sleep apnea syndrome (OSAS). SUBJECTS AND METHODS: Forty-seven patients suffering from OSAS were enrolled in this study. They were asked to answer a questionnaire about nasal stuffiness and mouth breathing, and the degree of tonsillar hypertrophy and shape of the oropharyngeal inlet were evaluated. The minimal cross-sectional area (MCA) of the nasal cavity was measured with acoustic rhinometry, followed by optimal CPAP level titration. We compared optimal CPAP levels with body mass index (BMI), perceived nasal stuffiness, and anatomic upper airway narrowing. RESULTS: The mean CPAP level was 6.32 ± 2.17 cm H2O. We found a significant correlation between CPAP levels and BMI (P = .01), apnea-hypopnea index (P = .015), and the degree of tonsillar hypertrophy (P = .011). Furthermore, the CPAP level increased significantly with the MCA on the narrow side of the nasal cavity (P = .038) in patients with a BMI less than 25. Other variables, such as subjective symptoms and anatomic nasal airway occlusion in obese patients (BMI ≥ 25), revealed no correlation with the CPAP level. CONCLUSIONS: Upper airway occlusion including nasal obstruction or tonsillar hypertrophy might increase the effective CPAP level. © The American Laryngological, Rhinological & Otological Society, Inc.