Catechol-O-methyl transferase (COMT) is an important estrogen-metabolizing enzyme, and common genetic variants in this gene could affect breast cancer risk. We conducted a large population-based case control study in Massachusetts, New Hampshire, and Wisconsin to examine six strategically selected COMT haplotype-tagging (ht) single nucleotide polymorphism (SNPs), including the val158met polymorphism (rs4680), in relation to breast cancer risk. Analyses were based on 1,655 Caucasian women with invasive breast cancer and 1,470 Caucasian controls. None of the six individual SNPs were associated with breast cancer risk. The global test for haplotype associations was nonsignificant (p-value=0.097), although two uncommon haplotypes present in 6% of the study population showed statistically significant inverse associations with risk. These results suggest that genetic variation in COMT has no significant association with breast cancer risk among Caucasian women.