To evaluate the role of genetic polymorphisms of ataxia telangiectasia mutated (ATM) in the etiology of breast cancer, a hospital-based case-control study was conducted in Korea. Nine-hundred ninety-six histologically confirmed incident breast cancer cases and 1,181 cancer-free controls were recruited in Seoul between 1995 and 2003. Genotypes of the ATM polymorphisms-5144A>T, IVS21+1049T>C, IVS33-55T>C, IVS34+60G>A, and 3393T>G were determined by the 5′-nuclease assay. Individual haplotypes were estimated from genotype data by a Bayesian method. Five ATM alleles were found to be in strong linkage disequilibrium (D′ > 0.82; P < 0.001). Haplotype frequencies were significantly different between cases and controls (χ2 test, P < 0.001). The ATM IVS21+1049 TC or CC, IVS34+60 GA or AA, and 3393 TG or GG genotypes were associated with increased breast cancer risk, particularly in premenopausal women [odds ratios (OR), 1.51; 95% confidence interval (CI), 1.11-2.05; OR, 1.42; 95% CI, 1.08-1.88; and OR, 1.37; 95% CI, 1.04-1.80, respectively]. Compared with diploid of TCCAG:TCCAG, the most common haplotype, the ATTGT:ATTGT was associated with decreased risk of breast cancer with borderline significance (OR, 0.77; 95% CI, 0.58-1.04) and TCCAG:ATCGT and ATTGT:ACCAG were associated with increased breast cancer risk (OR, 2.30; 95% CI, 1.18-4.48 and OR, 2.43; 95% CI, 1.1.07-5.52, respectively) after adjusting for age, education, age at first full-term pregnancy, parity, family history of breast cancer, alcohol consumption, and smoking. As the number of ATTGT haplotype decreased, the risk of breast cancer increased (P for trend <0.01). Our results thus suggest that genetic polymorphisms of ATM play an important role in the development of breast cancer in Korean women.