OBJECTIVE: We tested the hypotheses that Prothrombin G20210A heterozygosity associate with increased risk of venous thromboembolism (VTE), ischemic heart disease (IHD), and ischemic cerebrovascular disease (ICVD) in the general population and re-tested risk of IHD and ICVD in two case-control studies. METHODS: 9231 individuals from the Danish general population were followed for VTE (VTE=DVT+PE), deep venous thrombosis (DVT), pulmonary embolism (PE), IHD, myocardial infarction (MI), ICVD, and ischemic stroke (IS) for a median of 24 years. Case-control studies included 2461 IHD cases and 867 ICVD cases. RESULTS: In the general population, Prothrombin G20210A heterozygotes had1.3 (95% CI:0.6-2.9) fold risk for VTE, 0.6 (0.2-2.0) for DVT, 1.7(0.6-4.8) for PE, 1.5(1.1-2.1) for IHD, 1.7(1.1-2.7) for MI, 1.1(0.6-1.9) for ICVD, and 1.1(0.5-2.1) for IS compared to non-carriers. Double heterozygotes for Prothrombin G20210A and Factor V Leiden versus double non-carriers had a multifactorially adjusted hazard ratio for IHD of 6.0(2.0-19). In case-control studies, multifactorially adjusted odds ratios for Prothrombin G20210A heterozygotes versus non-carriers were 2.0(1.1-3.4) for IHD, 2.0(1.0-3.8) for MI, 1.4(0.7-3.1) for ICVD, and 2.1(0.8-5.4) for IS. CONCLUSION: Prothrombin G20210A heterozygosity alone and in combination with Factor V Leiden R506Q heterozygosity predicts 1.5 and 6.0 fold risk of IHD compared to non-carriers.