The prevalence of acute coronary syndrome in Korea has steadily increased, however, the understanding of and participation rate in cardiac rehabilitation (CR) is very low. There are few studies have been conducted in myocardial infarction (MI) patients with reduced heart function, a so-called high-risk group. Therefore, it is necessary to compare the effects of CR on the degree of improvement in cardiopulmonary fitness (CPF), whether MI patients participate or not, especially in the patients that are at a high risk of MI. Three hundred and ninety-four patients that were commissioned for CR between January 2016 and December 2020 were screened for risk stratification based on the American Association of Cardiovascular and Pulmonary Rehabilitation guidelines, and 115 were classified as high-risk patients. We retrospectively reviewed the patients who underwent both an exercise tolerance test (ETT) during the initial visit and 3 months after the onset of the study. During this period, 42 subjects were included, of which, 26 underwent at least one CR session and 16 did not. The baseline characteristics of the patients showed no significant differences. The results of the CPF improvement were measured as peak oxygen consumption (VO2 peak) and metabolic equivalent of tasks (METs) values which were derived through the ETT. Prior to the ETT, all of the demographic features, including ejection fraction, showed that there were no significant differences between the two groups. The initial CPF values were the same. However, after three months, the VO2 peak and METs values showed that there were significant differences between the two groups (p < 0.01). Additionally, the exercise time differed significantly between the two groups. The CPF values and exercise time showed a significant increase after 3 months in the CR participants. Therefore, it is necessary to initiate cardiac rehabilitation especially in high-risk patients as soon as the patient’s vital signs are stable to improve their cardiopulmonary function.