OBJECTIVES: To determine the safety of intramyocardial injection of autologous bone marrow cells in patients undergoing surgical myocardial revascularization (CABG) for severe coronary artery disease. INTRODUCTION: There is little data available regarding the safety profile of autologous bone marrow cells injected during surgical myocardial revascularization. Potential risks include arrythmias, fibrosis in the injected sites and growth of non-cardiac tissues. METHODS: Ten patients (eight men) were enrolled; they were 59±5 years old with limiting angina and were non-optimal candidates for complete CABG. Bone marrow cells (1.3±0.3x10(8)) were obtained prior to surgery, and the lymphomonocytic fraction (CD34+=1.8±0.3%) was separated by density gradient centrifugation. During surgery, bone marrow cells were injected in non-grafted areas of ischemic myocardium. During the first year after surgery, the patients underwent laboratory tests, cardiac imaging, and 24-hour ECG monitoring. RESULTS: Injected segments: inferior (n=7), anterior (n=2), septal (n=1), apical (n=1), and lateral (n=1) walls. Except for a transient elevation of C-reactive protein at one month post-surgery (P=0.01), laboratory tests results were within normal ranges; neither complex arrhythmias nor structural abnormalities were detected during follow-up. There was a reduction in functional class of angina from 3.6±0.8 (baseline) to 1.2±0.4 (one year) (P<0.0001). Also, patients had a significant decrease in the ischemic score assessed by magnetic resonance, not only globally from 0.65±0.14 (baseline) to 0.17±0.05 (one year) (P=0.002), but also in the injected areas from 1.11±0.20 (baseline) to 0.34±0.13 (one year) (P=0.0009). CONCLUSIONS: Intramyocardial injection of bone marrow cells combined with CABG appears to be safe. Theoretical concerns with arrhythmias and/or structural abnormalities after cell therapy were not confirmed in this safety trial.