© 2016 by the American Thoracic Society. Rationale: Seasonal nadirs in 25-hydroxyvitamin D (25[OH]D) concentrations overlap with increased incidence and severity of obstructive sleep apnea (OSA) in winter. We hypothesized that, because lower 25(OH)D concentrations might lead to upper airway muscle dysfunction, low 25(OH)D would be associated with higher apnea-hypopnea index (AHI), a measure of OSA severity. Objectives: To determine if lower 25(OH)D concentration is associated with greater prevalence and increased severity of OSA, independent of established OSA risk factors. Methods: Using unconditional logistic regression, we performed a cross-sectional analysis in the Outcomes of Sleep Disorders in Older Men study, which included in-home overnight polysomnography, serum 25(OH)D measurement, and collection of demographic and comorbidity data. The primary outcome was severe sleep apnea, as defined by AHI of 30/h or more. Measurements and Main Results: Among 2,827 community dwelling, largely white (92.2%), elderly (aged 76.465.5 yr [mean6SD]) males, mean 25(OH)D concentration was 28.8 (68.8) ng/ml. Subjects within the lowest quartile of 25(OH)D (6-23 ng/ml) had greater odds of severe sleep apnea in unadjusted analyses (odds ratio = 1.45; 95% confidence interval = 1.02-2.07) when compared with the highest 25(OH)D quartile (35-84 ng/ml). However, further adjustment for established OSA risk factors strongly attenuated this association (multivariable adjusted odds ratio = 1.05; 95% confidence interval = 0.72-1.52), with body mass index and neck circumference as the main confounders. There was also no evidence of an independent association between lower 25(OH)D levels and increased odds ofmild (AHI = 5.0-14.9/h) or moderate (AHI = 15.0-29.9/h) sleep apnea. Conclusions: Amongcommunity-dwellingolder men, the association between lower 25(OH)D and sleep apnea was largely explained by confounding by larger body mass index and neck circumference.