A best evidence topic in cardiac surgery was written according to a structured protocol. The question addressed was 'Can intracoronary stem cell injection permanently improve cardiac function after myocardial infarction?'. Altogether 314 papers were found using the reported search, of which five represented the best evidence to answer the clinical question. The authors, journal, date and country of publication, patient group studied, study type, relevant outcomes and results of these papers are tabulated. We conclude that stem cells may have controversial effects on cardiac function in long-term follow-up of more than two years as they improved the left ventricular ejection fraction and end systolic volume index just in two studies in which none of them utilized cardiac magnetic resonance imaging (MRI), as the most reliable method, to quantify cardiac function. However, all remaining three trials which reported negative results used cardiac MRI for assessment of cardiac indexes which may be more precise and accurate than echocardiographic assessments. So the reliability of the positive trials is lower than negative resulted trials in terms of cardiac function assessment method. Stem cell therapy almost always offered short-term benefits over the best medical treatment, but the long-term benefits are still a matter of debate.