Cell therapy is an evolving option for patients with end-stage heart failure. First-generation cell therapy trials have had marginal success. Our goal was to evaluate retrograde delivery of allogeneic umbilical cord subepithelial cells (UCSECs) in patients with heart failure. A prospective open-label dose escalation study of the safety and feasibility of UCSECs infused retrogradely into the coronary sinus was performed. Patients received a single dose of either 100 million (M), 200M, or 400M cells. The patients were followed for 2 years. Twenty-four patients were successfully enrolled in the study. The patients had UCSEC infusion without procedure-related complications. The ejection fraction in patients receiving UCSECs demonstrated improvement compared to baseline; from 25.4% (±5.5) at screening to 34.9% (±4.1) at 12 months. End-systolic diameter decreased significantly from 59.9 (±5.3) mm to 52.6 (±2.7) mm (p < 0.05). Retrograde UCSEC delivery was safe and feasible in all three dosage groups. Patients receiving 200M and 400M UCSECs showed signs of early improvement in left ventricular ejection fraction (LVEF) and remodeling. This study provides the basis for a larger clinical trial in heart failure (HF) patients using the middle or high dose of UCSECs.