Recent discoveries in the physiology and pathology of myocardial progenitor cells have allowed researchers to better understand a variety of cardiac pathologies and look at the pathophysiology of arrhythmias from a different perspective. Since the myocardium is composed of a syncytium of electrically interconnected cells, the process of incorporation of newly formed or imported cells into its structure is particularly important. Progenitor cells are stimulated by spontaneous electrical activity, a lengthy action potential, and easily induced triggering activity. All these can lead to arrhythmias development via the three classical mechanisms (reentrancy, automatism, or triggering activity). Transplanted stem cells can influence the electrophysiological properties of cardiomyocytes, thus creating a proarrhythmic substrate. The islets of unbound cells can form an anatomical block, causing unidirectional blockages and recurrent arrhythmias. Similarly, stem cells are capable of establishing heterotopic excitation foci with cardiac stimulatory activity. Finally, the paracrine factors produced by stem cells can also cause proarrhythmic effects. The review examines the factors that influence the proarrhythmic properties of administered stem cells and the mechanisms of arrhythmia development. The results indicate that further research should be carried out to establish the possible impact of stem cells on the development of arrhythmias.