The existing longitudinal cohort data indicate that CPAP treatment is associated with a reduction in cardiovascular risk among patients with sleep apnea compared with patients who are not treated with CPAP even after adjustment for vascular risk factors and that this finding is most robust for patients with the most severe sleep apnea.737 739 For example, a study of 264 healthy subjects, 403 untreated patients with sleep apnea, and 372 patients with CPAP treatment for 10 years739 had a combined vascular event end point that included fatal or non- fatal stroke or MI or acute coronary syndrome requiring car- diac intervention. In this cohort, severe untreated sleep apnea was associated with a 3-fold increased risk of vascular events (adjusted OR, 2.87; 95% CI, 1.17 7.51 for cardiovascular death; OR, 3.17; 95% CI, 1.12 7.52 for nonfatal cardiovascu- lar events), but patients with treated sleep apnea had vascular Meschia et al Guidelines for the Primary Prevention of Stroke 3791 event risks that were similar to those of patients with mild untreated sleep apnea and healthy subjects. A cardiovascular end-point benefit was observed with CPAP treatment among 364 patients receiving CPAP compared with 85 untreated The adjusted HR was 0.34 (95% CI, 0.20 0.58) for CPAP treatment. 