Matrix metalloproteinase-3 (MMP-3) degrades the extracellular matrix and may contribute to the weakening of the plaque cap. To determine whether genotype-phenotype associations differed in different categories of acute coronary syndrome, we enrolled 650 consecutive Taiwanese patients diagnosed with acute coronary syndrome. Genotypic analysis was done on DNA using polymerase chain reaction and direct sequencing on the 5 adenines (5A)/6 adenines (6A; -1,171 bp) polymorphism in the MMP-3 gene promoter region. The frequency of the 5A polymorphism was higher in patients with acute coronary syndrome, especially in those with ST-elevation myocardial infarction (p <0.01). The number of 5A allele polymorphisms was strongly associated with more complex coronary angiography (diffuse score for 5A/5A vs 5A/6A vs 6A/6A, 6.6 +/- 1.2 vs 5.3 +/- 1.3 vs 4.6 +/- 1.1, all p values <0.05 in subgroup analysis) and higher plasma MMP-3 activity in this acute coronary syndrome cohort (MMP-3 level for 6A/6A vs 5A/6A vs 5A/5A, 21.0 +/- 2.2 vs 23.3 +/- 2.1 vs 27.9 +/- 2.2 ng/ml, all p values <0.05 in subgroup analysis). Multiple logistic regression analysis showed that this polymorphism, in addition to hypertension, diabetes, and a history of smoking, was an independent risk factor (odds ratio 2.2, 95% confidence interval 1.1 to 4.3, p = 0.02) for the occurrence of acute coronary syndrome. Further, carriers of this polymorphism who smoked had a significantly increased (20-fold) risk of acute coronary syndrome compared with nonsmoking noncarriers. In conclusion, the MMP-3 5A/6A polymorphism is significantly associated with the occurrence of acute coronary syndrome, MMP-3 activity, and severity of coronary atherosclerosis. There is a synergistic effect between smoking and this genetic risk factor for acute coronary syndrome.