Allele frequencies for the insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene were determined in a large case-control study of 517 unselected patients with venous thromboembolism and 478 blood donors. The D allele frequency was 0. 53 [95% confidence interval (CI) 0.50-0.56] in patients and 0.54 (95% CI 0.50-0.57) in controls, giving an odds ratio for the D allele of 0.97 (95% CI 0.81-1.16). In the same population, the odds ratio for the factor V Leiden mutation (F5G1691A) was 6.9 (95% CI 4. 0-11.9). Therefore, the ACE I/D polymorphism is not a risk factor in a representative group of unselected patients with venous thromboembolism. The possibility that the I/D polymorphism is a risk factor for venous thromboembolism specifically after hip replacement cannot be excluded.