To examine the effect of untreated obstructive sleep apnea (OSA) on health care utilization (HCU) and costs among a nationally representative sample of Medicare beneficiaries.Our data source was a random 5% sample of Medicare administrative claims data for years 2006-2013. OSA was operationalized as (1) receipt of one or more International Classification of Disease, Version 9, Clinical Modification diagnostic codes for OSA in combination with (2) initiation of OSA treatment with either continuous positive airway pressure or oral appliance (OA) therapy. First, HCU and costs were assessed during the 12 months prior to treatment initiation. Next, these HCU and costs were compared between beneficiaries with OSA and matched control patients without sleep-disordered breathing using generalized linear models.The final sample (n = 287,191) included 10,317 beneficiaries with OSA and 276,874 control patients. In fully adjusted models, during the year prior to OSA diagnosis and relative to matched control patients, beneficiaries with OSA demonstrated increased HCU and higher mean total annual costs ($19,566, 95% confidence interval [CI] $13,239, $25,894) as well as higher mean annual costs across all individual points of service. Inpatient care was associated with the highest incremental costs (ie, greater than control patients; $15,482, 95% CI $8,521, $22,443) and prescriptions were associated with the lowest incremental costs (ie, greater than control patients; $431, 95% CI $339, $522).In this randomly selected and nationally representative sample of Medicare beneficiaries and relative to matched control patients, individuals with untreated OSA demonstrated increased HCU and costs across all points of service.