Obstructive sleep apnea (OSA) is a common form of sleep-disordered breathing affecting up to 15% of the population in North America. Patients with OSA may be at increased risk for cardiovascular diseases such as hypertension, coronary artery disease, stroke, congestive heart disease, pulmonary hypertension, and arrhythmias. The relationship between OSA and cardiovascular disease is complex, and an understanding of the mechanisms that are responsible for these proposed relationships continues to evolve. Some evidence suggests that intermittent hypoxemia-induced endothelial dysfunction and sympathetic overdrive may mediate the deleterious cardiovascular effects of OSA. In recent years, several risk factors and abnormal levels of serum biomarkers have been associated with untreated OSA; these serum levels normalize with effective treatment of OSA. Serum markers that have been studied in OSA include C-reactive protein (CRP), IL-6, fibrinogen, endothelin-1, nitric oxide (NO), leptin, natriuretic peptides (ANP and BNP), adenosine, uric acid, and homocysteine. Abnormal levels of these biomarkers are seen predominantly in patients with moderate to severe OSA, and in patients with cardiovascular complications. Although the clinical utility of these serum biomarkers has not been fully elucidated, monitoring their levels may have potential in determining which OSA patients are at higher risk for cardiovascular complications, and thus help physicians to determine which patients warrant more intensive treatment.