We describe the appearance of delayed episodes of ventricular arrhythmias in 4 patients out of 72 undergoing intracoronary transplantation of autologous bone marrow mononuclear cells (BMMC) following ST elevated myocardial infarction (STEMI). Two cases with severely depressed systolic function presented electrical storms with monomorphic sustained ventricular tachycardia (SVT) within 2 to 3 days following cell transplantation, even though there were no periprocedural complications. Both patients were implanted with an internal defibrillator (ICD) after ruling out coronary re-occlusion. The remaining 2 patients presented several asymptomatic episodes of non-sustained ventricular tachycardia within one month following cell transfer. Only one of the latter presented syncopal SVT through programmed ventricular stimulation, undergoing ICD implantation afterwards. Neither new arrhythmic episodes nor ICD interventions have occurred during later follow-up of the three ICD patients (639+/-59 days). Information from large multicenter databases and our historical cohort of STEMI patients indicates that the rate of VT occurring within the first weeks after the initial 48 hours of infarction is significantly lower than that observed in our cell-therapy experience. The lack of information regarding the appearance of malignant arrhythmias in patients with severe systolic dysfunction following this type of therapy after STEMI requires us to be extremely cautious. However, any claim of a mechanism related to cell transfer would be completely speculative with the available data. Therefore, our only aim when reporting our findings is to recommend a short but longer stay (2-3 days) following cell transplantation, particularly in patients with a natural tendency to develop arrhythmic events.