Cardiac stem cell therapy with bone-marrow-derived stem cells is a promising approach to facilitate myocardial regeneration after acute myocardial infarction or in congestive heart failure. The clinical data currently available seem to indicate that this approach is safe and is not associated with an increase in the number of adverse clinical events; nevertheless, the level of safety confidence is limited because of the small number of patients who have been treated and the absence of long-term clinical follow-up data. In order to establish the clinical safety of cardiac stem cell therapy, it will be necessary to collect additional data from both previous and ongoing clinical trials in subsets of patients relative to their background risk. Several conceptual safety concerns should also be addressed. These concerns relate to a number of operational mechanisms and include biological effects on differentiation, remote homing of transplanted stem cells, progression of atherosclerosis, and arrhythmias. The proactive scrutiny of these phenomena could eventually facilitate the translation of the promise of cardiac regeneration into a safe and effective therapy.