Background Granulocyte colony-stimulating factor (G-CSF) seems to improve cardiac function and perfusion when used systemically through mobilization of stem cells into peripheral blood, but results of previous clinical trials remain controversial. This study was designed to investigate safety and efficacy of subcutaneous injection of G-CSF on left ventricular function in patients with impaired left ventricular function after ST-segment elevation myocardial infarction (STEMI).Methods Thirty-three patients (22 men; age, (68.5±6.1) years) with STEMI and with comorbidity of leukopenia were included after successful primary percutaneous coronary intervention within 12 hours after symptom onset. Patients were randomized into G-CSF group who received G-CSF (10 μg/kg of body weight, daily) for continuous 7 days and control group. Results of blood analyses, echocardiography and angiography were documented as well as possibly occurred adverse events.Results No severe adverse events occurred in both groups. Mean segmental wall thickening in infract segments increased significantly at 6-month follow up compared with baseline in both groups, but the longitudinal variation between two groups had no significant difference (P >0.05). The same change could also be found in longitudinal variation of wall motion score index of infarct segments (P >0.05). At 6-month follow-up, left ventricular end-diastolic volume of both groups increased to a greater extent, but there were no significant differences between the two groups when comparing the longitudinal variations (P >0.05). In both groups, left ventricular ejection fraction measured by echocardiography ameliorated significantly at 6-month follow-up (P <0.05), but difference of the longitudinal variation between two groups was not significant (P >0.05). When pay attention to left ventricular ejection fraction measured by angiocardiography,difference of the longitudinal variation between groups was significant (P=0.046). Early diastolic mitral flow velocity deceleration time changed significantly at 6-month follow-up in both groups (P=0.05).Conclusions Mobilization of stem cells by G-CSF after reperfusion of infarct myocardium is safe and seems to offer a pragmatic strategy for recovery of myocardial global function.