Objective To prospectively evaluate the safety and therapeutic efficacy ofdalteparin in patients with high risk non-ST-elevation acute coronary syndromes (ACS) during percutaneous coronary intervention (PCI). Methods Atotal of 175 patients with high risk non-ST-elevation ACS were randomly assigned to 2 groups [dalteparin group and unfractionated heparin (UFH) group]. The patients in dalteparin group were given dalteparin at a dose of 5,000U subcutaneously soon after diagnosis and then an additional 60U/ kg intravenous bolus ofdalteparin before emergent PCI. Vascular access sheaths were removed immediately after PCI or coronary artery angiography; the patients in UFH group were given UFH intravenously at a dose of 25mgjust before PCI and an additional 65mg bolus was administered if angiographic findings showed that the patients were suitable for percutaneous transluminai coronary angioplasty (PTCA). Sheaths were removed at 4-6 hours after PCI; Results Eighty-three patients in dalteparin group underwent PCI while 82 patients in UFH group underwent PCI; anti-Xa activities of 52 patients in dalteparin group were measured. The average anti-Xa activity was (0.83±0.26) U/ml at 15 minutes after intravenous injection of dalteparin and anti-Xa>0.5U/ml was obtained in 96.1% of the patients; hematomas at puncture sites were significantly fewer in dalteparin group as compared with UFH group (2.3% vs 9. 2%, P < 0.05); none of the patients in 2 groups suffered major bleeding events. No death, acute arterial reocclusion or emergent revascularization events occurred at 30 days after PCI. Conclusions Our study demonstrated that early subcutaneous injection of dalteparin at a dose 5,000U after diagnosis and an additional 60U/kg intravenous bolus ofdalteparin before PCI is safe and efficacious for patients with high risk non-ST-elevation ACS undergoing emergent PCI