Patients with obstructive sleep apnea (OSA) are at increased risk for cardiovascular disease. Altered cardiovascular variability is a prognostic indicator for cardiovascular events. This review examines the evidence that OSA is accompanied by alterations in cardiovascular variability. This alteration is evident even in the absence of hypertension, heart failure or other disease states, and may be linked to the severity of OSA. The presence of clear-cut abnormalities in time and frequency-domain measures of blood-pressure and heart-rate variability in normotensive OSA patients provides intriguing evidence for the concept of an etiologic interaction between sleep apnea and cardiovascular disease. Mechanisms that could contribute to altered cardiovascular variability in patients with sleep apnea include abnormalities in chemoreflex, baroreflex and endothelial function.