Background: Autologous bone marrow mononuclear cell (ABMMNC) therapy has shown promise in patients with heart failure (HF). Cell function analysis may be important in interpreting trial results. Methods: In this prospective study, we evaluated the safety and efficacy of the transendocardial delivery of ABMMNCs in no-option patients with chronic HF. Efficacy was assessed by maximal myocardial oxygen consumption, single photon emission computed tomography, 2-dimensional echocardiography, and quality-of-life assessment (Minnesota Living with Heart Failure and Short Form 36). We also characterized patients' bone marrow cells by flow cytometry, colony-forming unit, and proliferative assays. Results: Cell-treated (n = 20) and control patients (n = 10) were similar at baseline. The procedure was safe; adverse events were similar in both groups. Canadian Cardiovascular Society angina score improved significantly (P = .001) in cell-treated patients, but function was not affected. Quality-of-life scores improved significantly at 6 months (P = .009 Minnesota Living with Heart Failure and P = .002 physical component of Short Form 36) over baseline in cell-treated but not control patients. Single photon emission computed tomography data suggested a trend toward improved perfusion in cell-treated patients. The proportion of fixed defects significantly increased in control (P = .02) but not in treated patients (P = .16). Function of patients' bone marrow mononuclear cells was severely impaired. Stratifying cell results by age showed that younger patients (≤60 years) had significantly more mesenchymal progenitor cells (colony-forming unit fibroblasts) than patients >60 years (20.16 ± 14.6 vs 10.92 ± 7.8, P = .04). Furthermore, cell-treated younger patients had significantly improved maximal myocardial oxygen consumption (15 ± 5.8, 18.6 ± 2.7, and 17 ± 3.7 mL/kg per minute at baseline, 3 months, and 6 months, respectively) compared with similarly aged control patients (14.3 ± 2.5, 13.7 ± 3.7, and 14.6 ± 4.7 mL/kg per minute, P = .04). Conclusions: ABMMNC therapy is safe and improves symptoms, quality of life, and possibly perfusion in patients with chronic HF.