Study Objectives: Erythrocyte levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (Omega-3 Index) were previously found to be associated with obstructive sleep apnea (OSA) at very low levels (< 5.0%) in only one epidemiologic study. OSA has comorbidities, such as arterial hypertension, heart failure, or major depression, also associated with a low Omega-3 Index. These comorbidities can be improved by increasing intake of EPA and DHA, and thus the Omega-3 Index, preferably to its target range of 8% to 11%. Symptoms of OSA might improve by increasing the Omega-3 Index, but more research is needed. Methods: In our sleep laboratory, 357 participants with OSA were recruited, and data from 315 participants were evaluated. Three categories of OSA (none/ mild, moderate, severe) were defined based on apnea-hypopnea index. Anthropometrics and lifestyle characteristics (smoking, alcohol, fish intake, omega-3 supplementation) were recorded. Erythrocyte fatty acid compositions were assessed with the HS-Omega-3 Index methodology. Results:The mean Omega-3 Index in all 3 categories of OSA was 5.7%, and no association with OSA was found. There were more male participants with severe OSA (79.7%, P = .042) than females, and participants with severe OSA had a significantly higher body mass index (32.11 ± 6.39 kg/m 2 , P = .009) than participants with mild or moderate OSA. Lifestyle characteristics were not significantly different. Conclusions:In contrast to our hypothesis, an Omega-3 Index of 5.7% was not associated with OSA severity. Previously, an Omega-3 Index < 5.0% was associated. Although our results suggest aiming for an Omega-3 Index > 5.7% in an intervention trial with EPA and DHA in OSA, comorbidities of OSA suggest a target range of 8% to 11%. Keywords: epidemiology, erythrocytes, hypopnea, omega-3 fatty acids Citation: Tittus J, Huber MT, Storck K, Köhler A, Köhler JM, von Arnim T, von Schacky C. Omega-3 Index and obstructive sleep apnea: a cross-sectional study. J Clin Sleep Med. 2017;13(10):1131-1136. I NTRO DUCTI O NSeveral risk factors, such as excess body weight, male sex, older age, neck circumference, predispose to obstructive sleep apnea (OSA).1-6 Conversely, OSA is an independent risk factor for arterial hypertension, heart failure, atrial fibrillation, or stroke. [1][2][3][4][5][6] In individuals with arterial hypertension or risk for cardiac disease, the prevalence of sleep apnea is 2 to 10 times higher than in individuals without the health issues mentioned.1-6 An adequate therapy of sleep apnea can decrease mortality and morbidity. [1][2][3][4][5][6] One epidemiologic study found low levels of omega-3 fatty acids in erythrocytes, as assessed with the HS-Omega-3 Index methodology, to be associated with OSA in United States adults.7 Based on this epidemiologic finding, and on the antiinflammatory effects of omega-3 fatty acids, it has been speculated that omega-3 fatty acids might be of therapeutic value in OSA, and intervention trials have been suggested. 8 This is further supported by many ...