© 2017 Sociedade Portuguesa de Pneumologia. Published by Elsevier España, S.L.U. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ; Introduction: Obstructive Sleep Apnea (OSA) is characterized by recurrent episodes of apnea and hypopnea, secondary to collapse of the upper airways during sleep. OSA is frequently associated to cardiovascular complications. In Portugal, its magnitude is unknown. Methods: In 2014 a cross-sectional study was performed using the Portuguese General Practitioner (GP) Sentinel Network (Rede Médicos Sentinela). Participants GP reported all OSA cases diagnosed and registered in their lists of users on the 31 December 2013. Frequency of OSA has been estimated by sex and age. OSA patients were also characterized by method of diagnosis, treatment, and underlying conditions. Association between risk factors and severe OSA (odds ratio) was calculated using a logistic regression model adjusting confounding. Results: Prevalence of OSA on the population aged 25 years or more was 0.89% (95 CI: 0.80---1.00%); it was higher in males 1.47% (95 CI: 1.30---1.67%) and in those aged between 65 and 74 (2.35%). Most had severe OSA (48.4%). Hypertension (75.9%), obesity (74.2%) and diabetes mellitus (34.1%) were the most frequent comorbidities. Being a male (OR: 2.6; 95 CI: 1.2---5.8) and having obesity (OR: 4.0; 95 CI: 1.8---8.6) were associated with an increased risk of severe OSA. Conclusion: Found frequency of OSA was lower than other countries estimates, which may be explained by differences on case definition but can also suggest underdiagnosis of this condition as reported by other authors. ; info:eu-repo/semantics/publishedVersion