Although common genetic variants in platelet collagen receptors influence platelet activation and thrombosis, the impact of polymorphisms in collagen genes on cardiovascular disease is unknown. To evaluate this, we genotyped a highly polymorphic intronic tandem repeat of the COL3A1 gene, encoding collagen type III, alpha 1. This revealed 4 common alleles (COL3A1-1, -2, -3, and -4). The 2 populations studied were as follows: (1) a cross-sectional study of 703 acute coronary syndrome (ACS) patients with myocardial infarction (MI) and unstable angina, and (2) a prospective study of 924 Caucasian patients from the OPUS (Orbofiban in Patients with Unstable coronary Syndromes)-TIMI-16 trial of the oral GPIIb/IIIa antagonist orbofiban. In addition, we studied 306 control subjects and 224 patients with stable angina. In the case-control population, COL3A1-4 carriers were protected against ACS (odds ratio [OR] = 0.57, 95% CI = 0.35-0.91, P =.02) and stable angina (OR = 0.35, 95% CI = 0.16-0.74, P =.006). In the OPUS population, allele 4 again appeared protective against composite end points (death, MI, stroke, recurrent ischemia, and urgent rehospitalization) (relative risk [RR] = 0.41, 95% CI = 0.17-1.00). There were significant interactions between COL3A1-1 and -3 variants and treatment. Allele COL3A1-3 was associated with an increased risk of the composite end point (RR = 1.65, 95% CI = 1.07-2.55) in patients randomized to orbofiban, but appeared protective in placebo patients (RR = 0.53, 95% CI = 0.28-0.98). We conclude that variants in the COL3A1 gene, the product of which is a vessel-wall protein and platelet ligand, modulate the risk of coronary artery disease and could also modulate the response to antithrombotic therapy. This is the first reported association between polymorphisms of extracellular matrix components and cardiovascular risk.