Rationale: Obstructive sleep apnea is linked to increased cardiovascular risk but the impact of mild forms of obstructive sleep apnea and their treatment on cardiovascular outcomes remains controversial. Objectives: To prospectively investigate cardiovascular outcomes in treated vs. untreated patients with obstructive sleep apnea. Methods: Consecutive sleep laboratory patients with all degrees of obstructive sleep apnea were included. Endpoints were non-fatal (myocardial infarction, stroke, and acute coronary syndrome requiring revascularization procedures) and fatal (death from myocardial infarction or stroke) cardiovascular events. Measurements: Comparison of event-free survival rates in treated vs. untreated patients (Kaplan-Meier estimates, log-rank test). Main results: Of 449 patients enrolled (age 56.0±10.5 yrs., BMI 30.8±5.4 kg/m²), 364 patients received obstructive sleep apnea-treatment, and 85 patients remained untreated. Median follow up was 72.0 months (1-156). Mean apnea-hypopnea-index (AHI) before treatment was 30.9±21.8/h in treated and 15.3±13.0/h in untreated patients, but there were no differences in cardiovascular comorbidities or risk factors. In patients with mild-moderate obstructive sleep apnea (n=288), events were more frequent in untreated patients (estimated event-free survival at 10 years: 51.8% vs. 80.3%, p<0.001, absolute risk reduction: 28.5%, number needed to treat to prevent one event/10 years: 3.5). After adjustment for age, gender, cardiovascular risk factors, and comorbidities at baseline obstructive sleep apnea-treatment was an independent predictor for events (hazard ratio: 0.36, 95%CI: 0.21-0.62, p<0.001). Conclusions: Obstructive sleep apnea-treatment was associated with a cardiovascular risk reduction of 64% independent from age and pre-existing cardiovascular comorbidities. Obstructive sleep apnea-treatment should be considered for primary and secondary cardiovascular prevention, even in milder obstructive sleep apnea.