Angiotensin II has been shown to have possible mitogenic and angiogenic effects in human cell lines and animal models of breast cancer. It is converted from its precursor to its active form by the angiotensin I-converting enzyme (ACE). A recent epidemiological study observed lower breast cancer incidence in female users of ACE inhibitors relative to nonusers with comparable cardiovascular conditions. To study the hypothesis that reduced ACE activity is associated with reduced risk of breast cancer, we conducted a nested case-control study within the Singapore Chinese Health Study Cohort to investigate the associations between the ACE A-240T and I/D gene polymorphisms, and breast cancer risk. For this analysis, 189 incident breast cancer cases and 671 female cohort control subjects were compared. The low-activity A and I alleles were the putative “low-risk” alleles. The I/D and A-240T genotypes exhibited significant linkage disequilibrium among Singapore Chinese (contingency coefficient = 0.74; P < 0.001). With adjustment for breast cancer risk factors, women with one or two copies of the low activity A allele showed a statistically significant reduction in risk compared with those with the TT genotype [odds ratio (OR), 0.55; 95% confidence interval (CI), 0.34–0.90]. The risk reduction was enhanced after excluding subjects with medical conditions for which ACE inhibitors are commonly prescribed (OR, 0.49; 95% CI, 0.27–0.89). Comparable results were obtained with respect to the I/D genotype and risk of breast cancer. When the I/D and A-240T genotypes were considered simultaneously, compared with women with the high-activity genotypes (either TT or DD or both), those with the low-activity genotypes (presence of both A and I alleles) exhibited lower breast cancer risk (OR, 0.46; 95% CI, 0.27–0.81). Our findings support experimental data implicating ACE and angiotensin II in breast cancer, and suggest that the renin-angiotensin system may serve as a therapeutic target for breast cancer treatment and prevention.