Background. Obstructive sleep apnea (OSA) is a common chronic disease with high impact both as an independent cardiovascular risk factor and in quality of life. Home positive pressure ventilation is the main available therapeutic intervention, depending on its utilization by patients. Objectives. Our aim was to determine the adherence to home ventilation therapy, prospecting for the determinants of compliance. Secondarily, we evaluated the direct costs of non-adherence. Material and methods. We conducted a cross-sectional study based on retrospective analysis of a sample of 1,183 OSA patients followed in a pulmonology department during 2018. Patients with less than 180 days of utilization were excluded. The adherence to ventilation therapy was defined by utilization of more than 4 hours a day, at least 70% of the days. Costs were calculated using Portuguese official prices for reimbursement. Results. A total of 744 patients were included for analysis. The adherents were 63.4%(95% CI: 60.5–67.4%). Ageing (p= 0.014), severity of disease at diagnosis (p= 0.044), room sharing (p= 0.002), working in the primary economic sector (p= 0.014) and using BiPAP (p= 0.046) were associated with higher adherence. The total costs of non-adherence were estimated at 112,373.68€. Conclusions. The adherence of patients to treatments is a relevant topic of discussion. Our results are concordant with literature, reinforcing the importance of holding the patient accountable to improve compliance with treatment. Adherence is lower than necessary to achieve the maximization of the therapeutic benefits, leading to a higher burden of disease, higher costs and significant waste.