Objective: Mask use has a substantial role in reducing the spread of COVID-19 in confined spaces. However, the use of masks is controversial in cases where intense effort is required, such as the treadmill exercise test. Since the majority of subjects undergoing this test are elderly individuals with comorbidities, the safety and reliability of the test are critical matters that must be elucidated in the post-COVID-19 era. In this study, the safety of the treadmill exercise test performed using a mask and the diagnostic reliability of the test were questioned. Method: In our clinic, before March 11, 2020, treadmill exercise tests were performed without a mask; whereas, after this date, patients were supplied with a standard surgical mask during the test. These two groups were compared with regard to various characteristics to assess diagnostic reliability and safety. Results: The diagnostic reliability and the safety of the stress test performed with a mask was found to be similar to that of tests performed without a mask. However, we noted that shortness of breath was more common in the mask-wearing group. Conclusion: The treadmill exercise test performed by wearing surgical masks is safe and its results are reliable for diagnosis.