Rationale. It is unknown whether obstructive sleep apnea (OSA) may be a risk factor for incident cardiovascular events in women. We sought to investigate whether OSA increases the incidence of a composite of stroke or coronary heart disease (CHD) in women, and the role of continuous positive airway pressure (CPAP) treatment on this association. Methods. Prospective, observational study conducted in two Spanish teaching hospitals between 1998 and 2007. Consecutive women referred for suspected OSA and free of previous stroke and CHD were analyzed. Women with an apnea-hypopnea index (AHI)<10 comprised the control group, and those with an AHI≥10 were diagnosed with OSA and classified as CPAP-treated (adherence≥4 hours/day) or untreated (adherence<4 hours/day or not prescribed). The follow-up ended in December 2010. Measurements and Results. A total of 967 women were studied (median follow-up 6.8 years, IQR 5.2-8.2). The untreated OSA group showed a greater incidence rate of the composite outcome than the control group (2.19 vs. 0.54 per 100 person-years, p<0.0005). Compared with the control group, the fully adjusted hazard ratios for the composite outcome incidence were 2.76 (95%CI, 1.35-5.62) for the untreated OSA group, and 0.91 (95%CI, 0.43-1.95) for the CPAP-treated group. When the type of cardiovascular event was separately assessed, untreated OSA showed a stronger association with incident stroke (adjusted HR 6.44, 95%CI 1.46-28.3) than with CHD (adjusted HR 1.77, 95%CI 0.76-4.09). Conclusion. In women, untreated OSA is associated with increased incidence of serious cardiovascular outcomes, particularly incident stroke. Adequate CPAP treatment seems to reduce this risk.