Background and Objectives: The purposes of this study were to compare the purchase rate and adherence regarding the use of two machines, continuous positive airway pressure (CPAP) and automatic positive airway pressure (APAP), and to determine associated factors affecting the purchase rate and adherence of PAP in the treatment of severe obstructive sleep apnea (OSA).Subjects and Method From March 2016 to December 2017, 45 patients diagnosed with severe OSA by polysomnography were enrolled in this study. Patients were prescribed CPAP or APAP by one doctor and allowed to decide whether to purchase the machine after a one-month lease period, which was identified as the purchase rate. Adherence was identified as using the machine 4 hight on 70% of nights following the PAP therapy of three months. Results: The adherence rate of CPAP (84%) was statistically and significantly higher than APAP (55%) (p=0.033). However, there was no significant difference in the adherence rate between CPAP (76%) and APAP (63%). The mean pressure and 95th percentile leak were 8.62 cm H2O and 12.97 L/min in the CPAP and 10.55 cm H2O and 20.36 L/min in the APAP, respectively. The mean pressure and 95th percentile leak were significantly lower in CPAP than in APAP (p=0.010 and p=0.014, respectively). Conclusion Although adherence was not significantly different, the purchase rate was significantly higher in the fixed CPAP than in APAP, which may have been influenced by high pressure and leak patients experienced when using the particular PAP machine.