The aims of the present study were to investigate the distribution of the insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene in breast cancer patients and the association between ACE genotypes and clinicopathologic features, as well as their effects on prognosis. We assessed the I/D polymophism of the ACE gene by using polymerase chain reaction from peripheral blood in breast cancer and healthy age-matched women. The clinicopathologic parameters of breast cancer patients were obtained from medical records. Of the 57 patients, 31 (54.4%) had DID, 24 (42.1%) had ID, and 2 (3.5%) had II genotypes. In control subjects, 33 (63.5%) had DD, 12 (23.1%) had ID, and 7 (13.4%) had II genotypes. The ID genotype was seen more commonly in breast cancer patients (p =.03). When the combination of ID and II genotypes was used as a reference group, the DD genotype was associated with negative hormone receptor status (p = .003), tumor size (p = .054), and lymph node involvement (p = .07) but not histologic high grade and c-erb B2 overexpression. These results suggest that the DID genotype may accompany poor prognostic factors and influence the tumor course.