Objective: To investigate the relationship of matrix metalloproteinase-9 polymorphism to acute coronary syndrome and its affect on the severity of coronary artery disease. Methods: By means of polymerase chain reaction(PCR) and restriction fragment length polymorphism, genotypes of 245 patients with acute coronary syndrome(ACS) and 205 healthy subjects were tested. Genotypes displaying C-1562T functional promoter polymorphism(of the MMP-9 gene) were determined. The relationship between the polymorphism of the MMP-9 gene and ACS and the severity of coronary vessels diseased was analyzed. Results: The frequency of C/T plus T/T genotypes and T allele in patients with ACS was significantly higher than that in healthy subjects(22.1% vs 12.7% and 11.4% vs 6.6% respectively). But they were not associated with the number of coronary arteries diseased. Conclusion: The MMP-9 polymorphism may be susceptible to ACS. But there was not significant difference between the AMI and UAP subgroups.