Recent studies have indicated that stem cell implantation increases cardiac function by repairing damaged myocardium. We investigated whether intracoronary transplantation of autologous bone marrow-derived mononuclear cells (BMMCs) confers beneficial effects in patients with refractory chronic heart failure. Twenty-eight patients received standard heart failure medication and BMMC transplantation (BMMC treatment) or standard medication only (controls). BMMCs were harvested from each patient. Clinical manifestations, biochemical assays, rhythm studies, echocardiograms, and positron emission tomograms were recorded. Fourteen patients with cell grafting had symptomatic relief of heart failure within 3 days. Left ventricular ejection fraction increased by 9.2% and 10.5% at 1 week and 3 months after the procedure, respectively, versus baseline (p < 0.01 for the 2 comparisons). Left ventricular end-systolic volume decreased by 30.7% after 3 months (p < 0.01). Brain natriuretic peptide levels at days 3 and 7 after cell infusion significantly decreased by 69.2% and 70.4%, respectively, whereas atrial natriuretic peptide levels increased by 30.1% at day 7. Positron emission tomographic analysis showed a significant increase in cell viability of 10.3% in the infarcted zone. No patient died in the BMMC-treated group at 6-month follow-up. In contrast, heart failure did not improve in any control patient. Left ventricular ejection fraction decreased by 7.2% after 3 months. Two control patients died from heart failure within 6 months. In conclusion, this is the first demonstration in humans that intracoronary BMMC transplantation is a feasible and safe therapeutic strategy to decrease symptoms, increase cardiac function, and possibly prolong life in patients with end-stage heart failure refractory to standard medical therapy.