Elderly women affected by obstructive sleep apnea (OSA) develop cognitive deficits compared to age-matched controls with normal sleep. This chapter reviews the postulated pathogenesis of cerebral small-vessel disease in subjects with obstructive sleep apnea. Patients with chronic nasal obstruction from allergies or anatomical defects of the nasal septum and turbinates tend to breathe through an open mouth during sleep and are more likely to develop OSA. The number of apneas and hypopneas per hour, the degree of oxygen desaturation, and the number of arousals and awakenings per hour constitute the main elements to quantify the severity of OSA. The mechanisms activated by OSA persist during daytime, along with the associated vascular, pulmonary, and endocrine responses. Obesity and the metabolic syndrome frequently occur in patients with OSA. Early recognition and treatment with continuous positive airway pressure (CPAP) will result in improvement of cognitive function and prevention of dementia.