Objective: Patients with silent and undiagnosed atrial fibrillation (AF) have increased risk of ischemic stroke. Patients with obstructive sleep apnea (OSA) have an increased risk of both AF and ischemic stroke. Our aim was to investigate the prevalence of silent AF and associated risk factors in patients investigated for OSA or with known OSA. Methods: This prospective observational study was performed in two sites; one outpatient sleep-clinic at Zealand University Hospital and one private Ear-Nose- and Throat clinic. Patients were investigated with a type-3 portable sleep-monitoring device, while heart rhythm was home-monitored for 7 days with an event-triggered loop recorder. Patients were stratified in groups of mild, moderate and severe OSA based on Apnea-Hypopnea-Index (AHI). Results: In a cohort of 303 patients, 238 (78.5%) were diagnosed with moderate/or severe OSA and 65 (21.5%) with no/mild OSA who constituted the control group. In 238 patients with moderate and severe OSA, AF was detected in 21 patients (8.8%) vs. 1 patient (1.5%,[p=0.045]) with mild OSA. Candidates for anticoagulation therapy were referred for further cardiovascular treatment. The majority of patients had known hypertension (n = 200,66%) and dyslipidemia (n = 235,[77.6%]) In patients with moderate/or severe OSA (AHI≥15), hypertension was more dysregulated (p=0.005) and more patients suffered from unknown prediabetes (n = 36, 3.1% vs. 14.3%[p<0.001]). Conclusion: Undiagnosed AF and undertreated cardiovascular modifiable risk factors are common in a cohort of patients with OSA. With this study we propose that long-period home-monitoring in these patients is useful for identifying candidates for preventive anticoagulation, cardiovascular treatment and possibly prevent future ischemic stroke.