Evidence is mounting that obstructive sleep apnea causes or contributes to many chronic medical diseases, and that treatment with continuous positive airway pressure (CPAP) often improves concomitant diseases. The author reviews the association of obstructive sleep apnea with arterial hypertension, pulmonary hypertension, stroke, coronary artery disease, heart failure, sudden death, gastroesophageal reflux disease, and diabetes mellitus.