To investigate a possible association between the human prothrombin gene G20210A polymorphism and coronary artery thrombosis, we screened 172 consecutive patients with ischaemic heart disease admitted for coronary arteriography. The patients were divided into two groups on the basis of their clinical history and examination of their hospital records: 66 patients with a definite previous myocardial infarction, and 106 with angina-like chest pain but no evidence of myocardial infarction. The overall frequency of the G20210A polymorphism was 0.011, four out of the 172 patients being heterozygous for the mutation. The allelic frequency was 0.015 in the group with myocardial infarction and 0.009 in the group without myocardial infarction (P = 0.622). The results of this study suggest that the single nucleotide polymorphism at position 20210 of the prothrombin gene is unlikely to be a risk factor for coronary thrombosis.