Background: Sleep apnea (SA) is associated with intermittent hypoxemia that may lead to ischemic preconditioning in the myocardium. This potential cardioprotective effect of SA may play a role in the development of non-ST-elevation myocardial infarction (NSTEMI) versus ST-elevation myocardial infarction (STEMI) during acute ischemia. However, there is limited evidence about the relative prevalence of NSTEMI versus STEMI in SA patients. We therefore prospectively investigated the prevalence of these two types of MI in patients with SA. Methods: We prospectively studied 782 consecutive patients admitted with the diagnosis of acute MI (both NSTEMI and STEMI). All subjects underwent sleep evaluations using a portable diagnostic device after at least 48 h post-admission, provided they were in stable condition. Six hundred and seven out of 782 patients had technically adequate sleep studies, and therefore were included in the final analysis. Results: SA was present in 65.7% (n=399) and NSTEMI in 30% (n=182) of patients. Increasing severity of SA was associated with increasing likelihood of NSTEMI, and with decreasing likelihood of STEMI (p < 0.001). Relative frequency of NSTEMI in the moderate to severe SA group (AHI >= 15 events/h) was 40.6% versus 29.9% for STEMI (p = 0.01). Conclusion: The prevalence of NSTEMI increases with increasing severity of SA. This finding may suggest a cardioprotective role of SA, which may attenuate the development of STEMI, perhaps through ischemic preconditioning. (C) 2017 Published by Elsevier Ireland Ltd.