The 6-min walking test is a practical, simple, inexpensive test, which does not require any exercise equipment or advanced training. The test has been proposed both as a functional status indicator and as an outcome measure in various categories of patients (postmyocardial infarction, heart failure, postcardiac surgery) admitted to rehabilitation programs. The purpose of this study is to review the literature regarding the usefulness of 6-min walking test for the evaluation of patients entering a cardiac rehabilitation program early after cardiac/thoracic surgery. The test is feasible and safe, even in elderly and frail patients, shortly after admission to an in-hospital rehabilitation program. The results of the test is influenced by many demographic and psychological variables, such as age, sex (with women showing lower functional capacity), comorbidity (particularly diabetes mellitus, arthritis, and other musculoskeletal diseases), disability, self-reported physical functioning, and general health perceptions; contrasting data correlate walked distance with left ventricular ejection fraction. Practical suggestions for test execution and results interpretation in this specific clinical setting are given according to current evidence.