['Obstructive sleep apnea (OSA) is linked to increased cardiovascular risk, but the impact of mild forms of OSA and their treatment on cardiovascular outcomes remains controversial.', 'To prospectively investigate cardiovascular outcomes in treated versus untreated patients with OSA.', 'Consecutive sleep laboratory patients with all degrees of OSA were included. Endpoints were nonfatal (myocardial infarction, stroke, and acute coronary syndrome requiring revascularization procedures) and fatal (death from myocardial infarction or stroke) cardiovascular events.', 'Comparison of event-free survival rates in treated versus untreated patients (Kaplan-Meier estimates, log-rank test). Of 449 patients enrolled (age, 56.0 +/- 10.5 years; body mass index, 30.8 +/- 5.4 kg/m(2)), 364 patients received OSA treatment, and 85 patients remained untreated. Median follow-up was 72.0 months (range, 1-156). Mean apnea-hypopnea index before treatment was 30.9 +/- 21.8/hour in treated and 15.3 +/- 13.0/hour in untreated patients, but there were no differences in cardiovascular comorbidities or risk factors. In patients with mild-moderate OSA (n = 288), events were more frequent in untreated patients (estimated event-free survival at 10 yr, 51.8 vs. 80.3% [P < 0.001]; absolute risk reduction, 28.5%; number needed to treat to prevent one event/10 yr, 3.5). After adjustment for age, gender, cardiovascular risk factors, and comorbidities at baseline, OSA treatment was an independent predictor for events (hazard ratio, 0.36; 95% confidence interval, 0.21-0.62; P < 0.001).'] Does continuous positive airway pressure treatment of mild to moderate obstructive sleep apnea reduce cardiovascular risk?